Change markdown-content hr style diaspora#5987

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 diaspora#6016

Changelog.md

@@ -21,6 +21,7 @@
 * Remove zip-zip workaround gem [#6001](https://github.com/diaspora/diaspora/pull/6001)
 * Cleanup and reorganize image assets [#6004](https://github.com/diaspora/diaspora/pull/6004)
 * Replace vendored assets for facebox by gem [#6005](https://github.com/diaspora/diaspora/pull/6005)
+* Improve styling of horizontal ruler in posts [#6016](https://github.com/diaspora/diaspora/pull/6016)
 
 ## Bug fixes
 * Disable auto follow back on aspect deletion [#5846](https://github.com/diaspora/diaspora/pull/5846)

app/assets/stylesheets/stream_element.scss

@@ -109,7 +109,11 @@
   .collapsible {
     overflow: hidden;
     position: relative;
-
+    .markdown-content hr {
+      width: 85%;
+      margin: 0.8em auto;
+      border-top: 1px solid $border-grey;
+    }
     .expander {
       position: absolute;
       bottom: 0;