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/views/modqueue/_sidebar.html.erb

@@ -12,7 +12,7 @@
   <%= render "posts/partials/index/blacklist" %>
 
   <p id="modqueue-sidebar-status" class="sidebar-section">
-    <h6>Status</h6>
+    <h2>Status</h2>
     <ul>
       <li>
         <%= link_to "status:pending", modqueue_index_path(search: { tags: "status:pending" }) %>
@@ -33,7 +33,7 @@
   </p>
 
   <p id="modqueue-sidebar-uploaders" class="sidebar-section">
-    <h6>Uploaders</h6>
+    <h2>Uploaders</h2>
     <ul>
       <% @uploaders.each do |uploader, count| %>
         <li>
@@ -46,7 +46,7 @@
   </p>
 
   <p id="modqueue-sidebar-tags" class="sidebar-section">
-    <h6>Tags</h6>
+    <h2>Tags</h2>
 
     <%= render "tag_list", tags: @artist_tags %>
     <%= render "tag_list", tags: @copyright_tags %>