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/posts/show.html.erb

@@ -15,17 +15,17 @@
   </section>
 
   <section id="post-information">
-    <h1>Information</h1>
+    <h2>Information</h2>
     <%= render "posts/partials/show/information", :post => @post %>
   </section>
 
   <section id="post-options">
-    <h1>Options</h1>
+    <h2>Options</h2>
     <%= render "posts/partials/show/options", :post => @post %>
   </section>
 
   <section id="post-history">
-    <h1>History</h1>
+    <h2>History</h2>
     <ul>
       <li id="post-history-tags"><%= link_to "Tags", post_versions_path(search: { post_id: @post.id }) %></li>
       <li id="post-history-pools"><%= link_to "Pools", pool_versions_path(search: { post_id: @post.id }) %></li>