`Rails.application.assets` is only available when `config.assets.compile`
is true (which is false in production). So the old way with a separate
rake task doesn't work in production. But we can get the filename of the
precompiled file from `Rails.application.assets_manifest.assets`.
* snippet now in a separate JS file - compiled and uglified with the other assets
* popup gets centered in opening browser window
* publisher gets pre-filled with markdown-styled content
