Basic autocomplete for admin posts tags

Similar to the skins autocomplete: basic LIKE search, maximum MAX_RECENT items. Can be tweaked by further requests.
Rake task to destroy all unused admin post tags
Merge branch 'refs/heads/master' into 0.9.12-issue-3177-admintags
Merge branch 'refs/heads/master' into 0.9.12-issue-3177-admintags
Merge branch 'refs/heads/master' into updated-issue-3177-admintags
Merge pull request #775 from CristinaRO/issue-3177-admintags

Issue 3177 admintags