Before we have only owner_id to determine group owner
With multiple owners per group we should get rid of owner_id in group.
So from now @group.owner will always be nil but
@group.owners return an actual array of users who can admin this group

- remove admin kaminari theme. Use gitlab theme everywhere
- use bootstrap styled for gitlab admin themes
- dont reload page when change issue filter

* layout: add 'Groups' item to main_menu
* admin/dashboard: add a ui-box for groups
* projects and groups: remove nav tabs