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
Trying to get the test to untick the right Unreleaved checkbox and then check that the work is revealed. Not quite there.
Merge branch 'master' into Issue-2241-Individual-reveal-test
Worked out which Unrevealed checkbox to uncheck to get the right work to pass the test; added deferred message for Issue 2243 which prevents the revealed work from triggering an email.
Sync Gemfile.lock with current master copy.
3308 change label for work skins on new work form
Change it on edit multiple, too
3308 Commit the CSS you tested, self -- .dashboard > form
Merge pull request #775 from CristinaRO/issue-3177-admintags

Issue 3177 admintags
Merge pull request #910 from sarken/issue_3308

3308 change label for work skins on new work form
Corrected a path that was causing two steps in the gift exchange feature to fail
Merge pull request #1556 from ariana-paris/Fix_gift_exchange_feature

Fix challenge_giftexchange_feature
Merge pull request #1546 from ariana-paris/Issue-2241-Individual-reveal-test

Issue 2241 and 2243 - reinstate individual reveal test and disable only the broken step