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/specific/forum.scss

@@ -58,9 +58,3 @@ div#c-forum-topics {
     color: var(--muted-text-color);
   }
 }
-
-@media screen and (max-width: 660px) {
-  #c-forum-topics #a-show {
-    padding: 0 0.5em;
-  }
-}