Merge branch '25202-fix-mr-widget-content-wrapping' into 'master'

Fixed MR widget content wrapping for XS viewports

Centers the "Accept Merge Request" button within the MR widget (and adds a bit of right/left padding) for viewports smaller than 768px.

BEFORE:
![Screen_Shot_2016-12-02_at_8_53_01_AM](/uploads/fcdae5302a1407090483dd3cff868c51/Screen_Shot_2016-12-02_at_8_53_01_AM.png)

AFTER:
![Screen_Shot_2016-12-02_at_9_43_37_AM](/uploads/f7d1dce082b79d263f05cea3c90e870a/Screen_Shot_2016-12-02_at_9_43_37_AM.png)

cc: @markglenfletcher @annabeldunstone 
Closes #25202

See merge request !7900

app/assets/stylesheets/pages/merge_requests.scss

@@ -129,6 +129,11 @@
         margin-bottom: 4px;
       }
 
+      .btn-grouped {
+        float: none;
+        margin-right: 0;
+      }
+
       .accept-action {
         width: 100%;
         text-align: center;

changelogs/unreleased/25202-fix-mr-widget-content-wrapping.yml

+---
+title: Centered Accept Merge Request button within MR widget and added padding for
+  viewports smaller than 768px
+merge_request: 
+author: Ryan Harris