Merge branch 'scss-empty-rule' into 'master'

Enable the EmptyRule SCSS Linter

Prevents empty SCSS rules from being included in the SCSS.

Also fixes the handful of lints caught by the linter.

Regarding the `.gh` rule in monokai.scss, which was added in [this commit](https://gitlab.com/gitlab-org/gitlab-ce/commit/bf079c24afb8ad2991a4eaf60a71a7bc45dd775d), it appears to be empty in [the original file as well](https://github.com/richleland/pygments-css/blob/master/monokai.css). So I think it's fine to delete the selector.

cc: @jschatz1 

See merge request !3769

.scss-lint.yml

@@ -65,7 +65,7 @@ linters:
   
   # Reports when you have an empty rule set.
   EmptyRule:
-    enabled: false
+    enabled: true
   
   # Reports when you have an @extend directive.
   ExtendDirective:

app/assets/stylesheets/framework/files.scss

@@ -84,10 +84,6 @@
       }
     }
 
-    &.blob_file {
-
-    }
-
     &.blob-no-preview {
       background: #eee;
       text-shadow: 0 1px 2px #fff;

app/assets/stylesheets/framework/selects.scss

@@ -121,9 +121,6 @@
   }
 }
 
-.select2-container-multi .select2-choices .select2-search-choice {
-}
-
 .select2-drop-active {
   margin-top: 6px;
   font-size: 14px;

app/assets/stylesheets/highlight/monokai.scss

@@ -111,8 +111,6 @@
   .vg { color: #f8f8f2 } /* Name.Variable.Global */
   .vi { color: #f8f8f2 } /* Name.Variable.Instance */
   .il { color: #ae81ff } /* Literal.Number.Integer.Long */
-
-  .gh { } /* Generic Heading & Diff Header */
   .gu { color: #75715e; } /* Generic.Subheading & Diff Unified/Comment? */
   .gd { color: #f92672; } /* Generic.Deleted & Diff Deleted */
   .gi { color: #a6e22e; } /* Generic.Inserted & Diff Inserted */

app/assets/stylesheets/pages/graph.scss

@@ -18,9 +18,6 @@
 }
 
 .graphs {
-  .graph-author-commits-count {
-  }
-
   .graph-author-email {
     float: right;
     color: #777;