make jQuery UI look more like ownCloud

This fixes the LDAP settings looking out of place, and also adds a header.

It includes a front-port of @LukasReschke’s https://github.com/owncloud/core/pull/10998

Please test @blizzz @LukasReschke @owncloud/designers 

Please also test @raghunayyar @georgehrke if this works with the Calendar which also uses jQuery UI.