Hide avatar settings behind feature flag

In favor of #157

app/views/settings/_profile.html.erb

@@ -31,6 +31,7 @@
       <% end %>
     <% end %>
 
+    <% if Flipper.enabled?(:avatar_upload, current_user) %>
     <label class="block">
       <p class="font-bold mb-1">
         Avatar
@@ -56,6 +57,7 @@
         </div>
       </div>
     </label>
+    <% end %>
 
     <p class="mt-8 pt-6 border-t border-gray-200 text-right">
       <%= f.submit 'Save', class: "btn-md btn-blue w-full md:w-auto" %>