Move select2 from files_external to core

Backport of 412da87e6583375f98ef1bdd755c56caab14b8ac from master
Now using select2 for the groups excluded from sharing
Load select2 on admin page
Remove excludegroup.php

The ajax call is now using ajax/appconfig.php instead
Added select2 on the apps page

Moved setupGroupsSelect() from admin.js to a common settings.js
as OC.Settings.setupGoupsSelect().

Now using select2 as well on the apps page.
Fixed select2 for admin and apps page

Added explicit escaping.
Now internally using a pipe symbol as separator for select2, to make it
possible to use group names containing commas.
fit select2 to ownCloud input style
Add select2 cache for complete group list

To avoid making a server request every time the dropdown opens, the
whole list of groups are cached (from the last request):

Whenever the user types in a search term it will still send server
requests.