Standardize font sizes and heading tags (<h1>-<h6>) to be more
consistent across the site.
Changes:
* Introduce font size CSS variables and start replacing hardcoded font
sizes with standard sizes.
* Change header tags to use only one <h1> per page. One <h1> per page is
recommended for SEO purposes. Usually this is for the page title, like
in forum threads or wiki pages.
* Standardize on <h2> for section headers in sidebars and <h3> for
smaller subsection headers. Don't use <h4>-<h6>.
* In DText, make h1-h4 headers all the same size. Standard wiki style is
to ignore h1-h3 and start at h4.
* In DText, make h4-h6 the same size as the h1-h3 tags outside of DText.
* In the tag list, change the <h1> and <h2> tag category headers to <h3>.
* Make usernames in comments and forum posts smaller. Also change the
<h4> tag for the commenter name to <div class="author-name">.
* Make the tag list, paginator, and nav menu smaller on mobile.
* Change h1#app-name-header to a#app-name-header.
Refactor tag scripts to fix multiple issues:
* Errors during tag scripting didn't show the actual error message, just
a generic "There was an error updating post #NNN" message.
* The quick edit form didn't show any error messages at all on failure.
* Thumbnails didn't have all their data attributes properly updated
after the post was updated.
This changes it so that thumbnails have their html fully replaced after
updating. This has the side effect of removing event handlers bound
directly to the thumbnail. A `danbooru:post-preview-updated` event is
fired in case userscripts need to detect when thumbnails are updated.
Mark all tag <input>s with a `data-autocomplete` attribute, instead of
hardcoding a list of html IDs to autocomplete in javascript.
This way should be less error prone. It fixes autocomplete in several places:
* Autocomplete for the search box on /posts didn't work in the
responsive layout. This was because /posts has two search boxes that
both have the id `tags`: one in the normal sidebar, and one in the
responsive tag list. $("#tags") only initialized autocomplete on the
first one.
* Autocomplete didn't work on the aliases or implications pages. This
was due to selecting the wrong html ids.
