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/partials/index/_excerpt.html.erb

@@ -40,13 +40,13 @@
     <% end %>
   <% elsif post_set.pool.present? %>
     <% post_set.pool.tap do |pool| %>
-      <h4>
+      <h2>
         <%= pool.pretty_category %>:
         <%= link_to pool.pretty_name, pool_path(pool), :class => "pool-category-#{pool.category}" %>
         <% if pool.is_deleted? %>
           <span class="inactive">(deleted)</span>
         <% end %>
-      </h4>
+      </h2>
 
       <div id="description" class="prose">
         <%= format_text(post_set.pool.description) %>
@@ -57,10 +57,10 @@
       </p>
     <% end %>
   <% elsif post_set.favgroup.present? %>
-    <h4>
+    <h2>
       Favorite Group:
       <%= link_to post_set.favgroup.pretty_name, favorite_group_path(post_set.favgroup) %>
-    </h4>
+    </h2>
     Creator: <%= link_to_user post_set.favgroup.creator %>
   <% elsif post_set.has_blank_wiki? %>
     <p>There is currently no wiki page for the tag <%= link_to_wiki post_set.tag.pretty_name %>. You can <%= link_to "create one", new_wiki_page_path(wiki_page: { title: post_set.tag.name }), rel: "nofollow" %>.</p>