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/forum_post_votes/_vote.html.erb

@@ -5,10 +5,10 @@
 
 <li class="vote-score-<%= vote.vote_type %>">
   <% if policy(forum_post).votable? && vote.creator_id == CurrentUser.id %>
-    <%= link_to content_tag(:i, nil, class: "far #{vote.fa_class}"), forum_post_vote_path(vote, format: "js"), remote: true, method: :delete %>
+    <%= link_to forum_post_vote_icon(vote), forum_post_vote_path(vote, format: "js"), remote: true, method: :delete %>
     <%= link_to_user vote.creator %>
   <% else %>
-    <%= content_tag(:i, nil, class: "far #{vote.fa_class}") %>
+    <%= forum_post_vote_icon(vote) %>
     <%= link_to_user vote.creator %>
   <% end %>
 </li>