Merge branch 'css-indentation' into 'master'

Fix Indentation lints in CSS.

As discussed in #14299.

cc: @jschatz1 

See merge request !3312

app/assets/stylesheets/framework/mixins.scss

 /**
  * Generic mixins
  */
- @mixin box-shadow($shadow) {
+@mixin box-shadow($shadow) {
   -webkit-box-shadow: $shadow;
   -moz-box-shadow: $shadow;
   -ms-box-shadow: $shadow;

app/assets/stylesheets/pages/admin.scss

@@ -34,9 +34,9 @@
     background: #fff
    }
 
-   .visibility-levels {
-     .controls {
-       margin-bottom: 9px;
+  .visibility-levels {
+    .controls {
+      margin-bottom: 9px;
     }
 
     i {

app/assets/stylesheets/pages/xterm.scss

@@ -21,19 +21,19 @@
   $l-white: #fff;
 
   .term-bold {
-      font-weight: bold;
+    font-weight: bold;
   }
   .term-italic {
-      font-style: italic;
+    font-style: italic;
   }
   .term-conceal {
-      visibility: hidden;
+    visibility: hidden;
   }
   .term-underline {
-      text-decoration: underline;
+    text-decoration: underline;
   }
   .term-cross {
-      text-decoration: line-through;
+    text-decoration: line-through;
   }
 
   .term-fg-black {