Show avatars in ajax user selectbox

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/javascripts/users_select.js.coffee

 $ ->
   userFormatResult = (user) ->
-    if user.avatar
-      avatar = user.avatar.url
+    if user.avatar_url
+      avatar = user.avatar_url
     else if gon.gravatar_enabled
       avatar = gon.gravatar_url
       avatar = avatar.replace('%{hash}', md5(user.email))

app/assets/stylesheets/generic/selects.scss

@@ -78,3 +78,7 @@ select {
 .project-refs-form .select2-container {
   margin-right: 10px;
 }
+
+.ajax-users-dropdown .select2-search {
+  padding-top: 4px;
+}

lib/api/entities.rb

@@ -6,6 +6,12 @@ class User < Grape::Entity
       expose :is_admin?, as: :is_admin
       expose :can_create_group?, as: :can_create_group
       expose :can_create_project?, as: :can_create_project
+
+      expose :avatar_url do |user, options|
+        if user.avatar.present?
+          user.avatar.url
+        end
+      end
     end
 
     class UserSafe < Grape::Entity