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/static/search_timeout.html.erb

@@ -6,7 +6,7 @@
 
     <p>Your search took too long to execute and was cancelled.</p>
 
-    <h6>Why did this happen?</h6>
+    <h3>Why did this happen?</h3>
 
     <p>
       Some kinds of searches are slower than others. If your search is too slow
@@ -15,7 +15,7 @@
     </p>
 
     <% if params[:controller] == "posts" && params[:action] == "index" && params[:tags].present? %>
-      <h6>What can I do?</h6>
+      <h3>What can I do?</h3>
 
       <p>Try changing your search:</p>
 
@@ -38,7 +38,7 @@
     <% end %>
 
     <% if !CurrentUser.is_gold? %>
-      <h6>Search limits</h6>
+      <h3>Search limits</h3>
 
       <p>
         Members are limited to searches that take up to 3 seconds long. You can