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/index.html.erb

@@ -6,7 +6,7 @@
   <%= render "posts/partials/index/blacklist" %>
 
   <section id="tag-box">
-    <h1>Tags</h1>
+    <h2>Tags</h2>
     <%= @post_set.tag_list_html(current_query: params[:tags], show_extra_links: policy(Post).show_extra_links?) %>
   </section>
 

app/views/posts/partials/common/_search.html.erb

@@ -1,7 +1,7 @@
 <%# path, tags %>
 
 <section id="search-box">
-  <h1>Search</h1>
+  <h2>Search</h2>
   <%= form_tag(path, method: "get", id: "search-box-form") do %>
     <% if params[:random] %>
       <%= hidden_field_tag :random, params[:random] %>

app/views/posts/partials/index/_blacklist.html.erb

@@ -1,5 +1,5 @@
 <div id="blacklist-box" class="sidebar-blacklist">
-  <h1>Blacklisted (<%= link_to_wiki "help", "help:blacklists" %>)</h1>
+  <h2>Blacklisted (<%= link_to_wiki "help", "help:blacklists" %>)</h2>
   <ul id="blacklist-list" class="list-bulleted"></ul>
   <%= link_to "Disable all", "#", :id => "disable-all-blacklists", :style => "display: none;" %>
   <%= link_to "Re-enable all", "#", :id => "re-enable-all-blacklists", :style => "display: none;" %>

app/views/posts/partials/index/_edit.html.erb

@@ -1,5 +1,5 @@
 <div id="quick-edit-div" style="display: none;">
-  <h1>Edit</h1>
+  <h2>Edit</h2>
 
   <%= edit_form_for(:post, html: { id: "quick-edit-form" }) do |f| %>
     <%= f.input :tag_string, label: "Tags", as: :text, input_html: { "data-autocomplete": "tag-edit" } %>

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>

app/views/posts/partials/index/_mode_menu.html.erb

@@ -1,6 +1,6 @@
 <% if policy(Post).can_use_mode_menu? %>
   <section id="mode-box">
-    <h1>Mode</h1>
+    <h2>Mode</h2>
     <form action="/">
       <select name="mode">
         <option value="view">View</option>

app/views/posts/partials/index/_options.html.erb

@@ -1,5 +1,5 @@
 <section id="options-box">
-  <h1>Options</h1>
+  <h2>Options</h2>
   <ul>
     <% if policy(SavedSearch).create? %>
       <li><%= button_tag(tag.i(class: "fas fa-bookmark") + " Save search", id: "save-search", class: "ui-button ui-widget ui-corner-all sub") %></li>

app/views/posts/partials/index/_related.html.erb

@@ -1,5 +1,5 @@
 <section id="related-box">
-  <h1>Related</h1>
+  <h2>Related</h2>
   <ul id="related-list">
     <% if discover_mode? %> 
       <li id="secondary-links-posts-hot"><%= link_to "Hot", posts_path(:tags => "order:rank") %></li>

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>