Admin interface: manage meta_keys improvements

Squashed commit of the following:

commit 829b03d9b1fbdefacc11a6660b845338cfe883c4
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Tue Jun 17 12:05:02 2014 +0200

    Squashed commits:

    - ability to filter used/not used meta keys
    - not used meta key can be deleted

commit 101e8cad56b8c2bc57a0dc1046955f14bea70d3a
Author: Michał Podlecki <m.podlecki@op.pl>
Date:   Fri May 30 12:50:46 2014 +0200

    admin cannot edit the label of a metakey