html: standardize font sizes and heading tags.

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.

app/logical/tag_category.rb

@@ -25,11 +25,6 @@ class TagCategory
       @@short_name_mapping ||= Hash[Danbooru.config.full_tag_config_info.map { |k, v| [v["short"], k] }]
     end
 
-    # Returns a hash mapping for split_tag_list_html (presenters/tag_set_presenter.rb)
-    def header_mapping
-      @@header_mapping ||= Hash[Danbooru.config.full_tag_config_info.map { |k, v| [k, v["header"]] }]
-    end
-
     # Returns a hash mapping for related tag buttons (javascripts/related_tag.js.erb)
     def related_button_mapping
       @@related_button_mapping ||= Hash[Danbooru.config.full_tag_config_info.map { |k, v| [k, v["relatedbutton"]] }]