Use select2 for the groups excluded from sharing in admin page

First commit moves select2 to core
Second commit uses select2 in the admin page for the groups selection:

![groupselect](https://cloud.githubusercontent.com/assets/277525/4203985/69ec9dd8-3833-11e4-9f2d-a38cba14f57c.png)

Please review @schiesbn @owncloud/designers @butonic @MTRichards @LukasReschke 

Fixes https://github.com/owncloud/core/issues/10936
Fixes second part of https://github.com/owncloud/core/issues/9014

If you guys like it I'll do the same for the apps page, next.