css: switch mod dashboard, keyboard shortcuts pages to flexbox.

* Switch /static/keyboard_shortcuts from float-based layout to flexbox.
* Switch /moderator/dashboard from float-based layout to flexbox.
* Remove various .clearfix, `clear: both` rules.
* Remove dead .quick-mod rule.

app/javascript/src/styles/base/020_base.scss

@@ -128,6 +128,13 @@ table tfoot {
   max-width: 70em;
 }
 
+.column-container {
+  display: flex;
+
+  .column-expand { flex: 1; }
+  .column-shrink { flex: 0; }
+}
+
 ul.list-bulleted {
   list-style: inside disc;
 }