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/javascript/src/styles/specific/pools.scss

@@ -27,17 +27,9 @@ div#add-to-pool-dialog {
     margin-left: 1em;
     cursor: pointer;
   }
-
-  h1 {
-    font-size: $h3_size;
-  }
 }
 
 div#c-pools {
-  h1 {
-    font-size: $h2_size;
-  }
-
   textarea {
     height: 10em;
   }
@@ -50,10 +42,6 @@ div#c-pools {
 }
 
 div#c-pool-orders, div#c-favorite-group-orders {
-  h1 {
-    font-size: $h2_size;
-  }
-
   div#a-edit {
     ul.ui-sortable {
       list-style-type: none;