Multiple UI Improvements (#4069)

Minor contast fix for file uploading text.

Updating icon markup

Fixing icon for mentions

Removing irrelevant characters

Updating closing tag for mentions icon

webapp/sass/layout/_headers.scss

@@ -217,6 +217,12 @@
 .sidebar--left,
 .sidebar--menu,
 .admin-sidebar {
+
+    .divider {
+        border-top: 1px solid transparent;
+        margin: 0.5em 0;
+    }
+
     .team__header {
         @include legacy-pie-clearfix;
         padding: 9px 10px;

webapp/sass/layout/_post-right.scss

@@ -87,7 +87,7 @@
         }
 
         .post-right-comments-upload-in-progress {
-            color: #a8adb7;
+            @include opacity(.7);
             margin-right: 10px;
             padding: 6px 0;
         }