views: factor out FontAwesome icons.

Factor out FontAwesome icons into a set of helpers. This is so that it's
easier to keep track of which icons we're using and easier to change
icons globally.

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

@@ -7,6 +7,6 @@
       <%= hidden_field_tag :random, params[:random] %>
     <% end %>
     <%= text_field_tag("tags", tags, :id => tags_dom_id, :"data-shortcut" => "q", :"data-autocomplete" => "tag-query") %>
-    <button id="search-box-submit" type="submit"><i class="fas fa-search"></i></button>
+    <button id="search-box-submit" type="submit"><%= search_icon %></button>
   <% end %>
 </section>