Refactor sass/scss stylesheets to use Rails asset helpers

Now that sass-rails and the asset pipeline work, we should use the
asset helpers Rails gives us rather than hard-coding
url("/assets/filename.png") for loading assets from sass/scss
stylesheets.

app/assets/stylesheets/application.css.sass

@@ -165,7 +165,7 @@ header
 
   :background
     :color rgb(40,35,35)
-    :image url('/assets/header-bg.png')
+    :image image-url('header-bg.png')
 
   :z-index 1001
   :padding 6px 0
@@ -1513,7 +1513,7 @@ ul#landing_nav
 
 #diaspora_description
   :background
-    :image url('/assets/ball.png')
+    :image image-url('ball.png')
     :repeat no-repeat
     :position top center
 
@@ -1847,7 +1847,7 @@ ul#press_logos
       :background-color transparent
 
     .oembed
-      :background url('/assets/ajax-loader2.gif') no-repeat center center
+      :background image-url('ajax-loader2.gif') no-repeat center center
       :display inline-block
       :max-width 100%
 
@@ -1856,7 +1856,7 @@ ul#press_logos
         :cursor pointer
 
         .video-overlay
-          :background url('/assets/video-overlay.png') no-repeat center center
+          :background image-url('video-overlay.png') no-repeat center center
           :position absolute
           :top 0
           :left 0
@@ -2276,7 +2276,7 @@ ul.left_nav
 
     > a.aspect_selector
       :background
-        :image url("/assets/icons/check_yes_ok.png")
+        :image image-url("icons/check_yes_ok.png")
         :position 3px 50%
         :repeat no-repeat
 

app/assets/stylesheets/autocomplete.css.sass

@@ -41,7 +41,7 @@
   :overflow hidden
 
 .ac_loading
-  :background white url('/assets/ajax-loader.gif') right center no-repeat
+  :background white image-url('ajax-loader.gif') right center no-repeat
 
 .ac_odd
   :background-color #fafafa

app/assets/stylesheets/mobile.css.scss

@@ -12,7 +12,7 @@ a {
 
 body {
   background: {
-    image: url('/assets/hatched-bg.jpg');
+    image: image-url('hatched-bg.jpg');
     position: fixed;
     /* scale background image down for iOS retina display */
     size: 200px;
@@ -276,7 +276,7 @@ body {
   @include box-shadow(0,1px,2px,#333);
 
   background: {
-    image: url("/assets/header-bg-long.jpg");
+    image: image-url("header-bg-long.jpg");
   }
 
   width: 100%;
@@ -377,7 +377,7 @@ footer {
         right: 14px;
       }
       background: {
-        image: url("/assets/icons/arrow_down_small.png");
+        image: image-url("icons/arrow_down_small.png");
         position: center right;
         repeat: no-repeat;
       }
@@ -460,28 +460,28 @@ footer {
     left: 5px; };
 
   &.loading {
-    background-image: url("/assets/mobile-spinner.gif");
+    background-image: image-url("mobile-spinner.gif");
   }
 }
 
 .reshare_action {
-  background-image: url("/assets/icons/reshare_mobile.png");
+  background-image: image-url("icons/reshare_mobile.png");
   &.active {
-    background-image: url("/assets/icons/reshare_mobile_active.png");
+    background-image: image-url("icons/reshare_mobile_active.png");
   }
 }
 
 .like_action {
-  background-image: url("/assets/icons/heart_mobile_grey.png");
+  background-image: image-url("icons/heart_mobile_grey.png");
   &.active {
-    background-image: url("/assets/icons/heart_mobile_red.png");
+    background-image: image-url("icons/heart_mobile_red.png");
   }
 }
 
 .comment_action.image_link {
-  background-image: url("/assets/icons/pencil_mobile_grey_active.png");
+  background-image: image-url("icons/pencil_mobile_grey_active.png");
   &.inactive {
-    background-image: url("/assets/icons/pencil_mobile_grey.png");
+    background-image: image-url("icons/pencil_mobile_grey.png");
   }
 }
 

app/assets/stylesheets/new-templates.css.scss

@@ -87,7 +87,7 @@ $pane-width: 420px;
 
 @mixin dark-hatched-bg() {
   background-color: #444;
-  background-image: url("/assets/hatched-bg-dark.png");
+  background-image: image-url("hatched-bg-dark.png");
 }
 
 @mixin photo-shadow() {
@@ -515,7 +515,7 @@ body.idle {
 
     border-top: 1px solid #444;
     text-align: left;
-    background-image: url("../images/hatched-bg-dark.png");
+    background-image: image-url("hatched-bg-dark.png");
   }
 
   #new-post-comment-container {

app/assets/stylesheets/ui.css.sass

@@ -127,25 +127,25 @@ input.button
 
         &.selected
           :background
-            :image url('/assets/icons/check_yes_ok.png')
+            :image image-url('icons/check_yes_ok.png')
             :position 3px 1px
             :repeat no-repeat
 
         &.selected:hover
           :background
-            :image url('/assets/icons/check_yes_ok_white.png')
+            :image image-url('icons/check_yes_ok_white.png')
             /* bump top position up 1px to compensate for the 1px added top border */
             :position 3px 0px
 
         &.loading
           :background
-            :image url('/assets/ajax-loader.gif')
+            :image image-url('ajax-loader.gif')
             :repeat no-repeat
             :position 4px 2px
 
         &.loading:hover
           :background
-            :image url('/assets/ajax-loader_inverted.gif')
+            :image image-url('ajax-loader_inverted.gif')
             /* bump top position up 1px to compensate for the 1px added top border */
             :position 4px 1px