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/comments/partials/show/_comment.html.erb

@@ -17,12 +17,12 @@
            <% end %>
            data-is-voted="<%= comment.voted_by?(CurrentUser.user) %>">
     <div class="author">
-      <h4>
+      <div class="author-name">
         <%= link_to_user comment.creator %>
         <% if comment.is_deleted? %>
           (deleted)
         <% end %>
-      </h4>
+      </div>
       <%= link_to time_ago_in_words_tagged(comment.created_at), post_path(comment.post, anchor: "comment_#{comment.id}"), class: "message-timestamp" %>
     </div>
     <div class="content">