Fix various elements to use standard font sizes instead of ad-hoc sizes.
Noticeable changes:
* Tags in autocomplete are slightly smaller.
* The favorite heart icon on posts is slightly smaller.
* Pool titles on thumbnails in the pool gallery page are slightly bigger.
* The page footer is slightly smaller.
* Timestamps on comments and forum posts are very slightly smaller.
* "Pending"/"approved"/"rejected" labels on forum posts are very slightly smaller.
* 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.
Position <span class="hint"> elements to the right of normal <input>
elements and underneath <textarea> elements. Fixes form hints being
badly positioned on some pages.
