Merge branch 'fix-user-autocomplete' into 'master'

Fixes problem with user autocomplete URI 

```
Failed to load resource: net::ERR_NAME_NOT_RESOLVED
http://autocomplete/users.json?search=&per_page=20&active=true&current_user=false
```

Fixes #4118, #4136

See merge request !2136

app/assets/javascripts/users_select.js.coffee

@@ -117,5 +117,5 @@ class @UsersSelect
       callback(users)
 
   buildUrl: (url) ->
-    url = gon.relative_url_root + url if gon.relative_url_root?
+    url = gon.relative_url_root.replace(/\/$/, '') + url if gon.relative_url_root?
     return url