base: dropdown menu and mobile UI

* Makes admin menu looks embadded in dropdown. (closes #1994)

* Fixes messages dropdown.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
base: dropdown menu improvement

* Fixes class name for higlight of new messages.

* Adds min-width for admin panel.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>