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/modqueue/_post.html.erb

@@ -31,7 +31,7 @@
         <span>
           <span id="score-for-post-<%= post.id %>"><%= post.score %></span>
           <% if policy(PostVote).create? %>
-            (vote <%= link_to tag.i(class: "far fa-thumbs-up"), post_post_votes_path(score: "up", post_id: post.id), remote: true, method: :post %>/<%= link_to tag.i(class: "far fa-thumbs-down"), post_post_votes_path(score: "down", post_id: post.id), remote: true, method: :post %>)
+            (vote <%= link_to upvote_icon, post_post_votes_path(score: "up", post_id: post.id), remote: true, method: :post %>/<%= link_to downvote_icon, post_post_votes_path(score: "down", post_id: post.id), remote: true, method: :post %>)
           <% end %>
         </span>
       </span>

app/views/modqueue/_sidebar.html.erb

@@ -5,7 +5,7 @@
     <%= f.input :tags, label: false, input_html: { placeholder: "Tags", value: params.dig(:search, :tags), "data-autocomplete": "tag-query" } %>
     <%= f.input :order, label: false, collection: [["Newest", "modqueue"], ["Oldest", "modqueue_asc"], ["Score (Highest)", "score"], ["Score (Lowest)", "score_asc"]], selected: params[:search][:order] %>
     <%= f.button :button, name: nil, id: "search-box-submit" do %>
-      <i class="fas fa-search"></i>
+      <%= search_icon %>
     <% end %>
   <% end %>