css: clean up responsive css.

* Reduce size of buttons, form inputs, site menu items, paginator, and
  sidebar text.
* Remove various dead rules (nonexistent selectors, rules overriden by
  other rules, rules that were otherwise redundant or did nothing).
* Increase page margin size.
* Fix notes being mispositioned (caused by `#image { margin-top: 5px }` rule).

app/javascript/src/styles/common/main_layout.scss

@@ -52,7 +52,7 @@ div.clearfix {
   @media (max-width: 660px) {
     flex-direction: column;
 
-    #content { order: 1; }
+    #content { order: 1; padding: 0; }
     #sidebar { order: 2; }
   }
 }