The <hr> tag in the markdown-content was #eee,
and thus almost invisible. It has been changed to
$border-gray (currently #ddd) to be consistent
with the other <hr> tags.
Adjust markdown-content hr style further
The hr tag is now centered at 85% width. It also
adds a default vertical margin that is equivalent
to the 0.8em the paragraph tags provide.
Refactor scss for markdown-content hr
Closes#6016
