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
3806 add Twitter Javascript to chapters/show
3806 Move Twitter JavaScript to work navigation partial so it is included for chapters
3812 Admin post filter labels should select associated form elements
3812 Make Filters text on admin posts index translateable
3870 Tidy up admin settings page
3870 Not validating due to omitted li tag in admin header
3273 Extra commas when tags are hidden at preference level
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.
Fix for Rspec SMTP errors
Merge pull request #1235 from sarken/issue_3273

3273 Extra commas when tags are hidden at preference level
Merge pull request #1379 from sarken/issue_3806

3806 Missing title/fandom information when sharing multi-chapter works on Twitter
Merge pull request #1464 from sarken/issue_3870

3870 Tidy up admin settings page