Add custom headers for email messages

See http://meta.discourse.org/t/custom-headers-in-emails-site-settings/7480
Log all changes of user trust level by an admin
If the change doesn't go through, don't log anything
Prevent errors in case #{rails_root}/tmp does not exist

Dir.mkdir is not recursive. i.e - It doesn't do the equivalent of `mkdir -p` but just plain `mkdir`.
And that is a problem if #{rails_root}/tmp is missing. More here.
http://meta.discourse.org/t/keep-getting-the-same-error-every-start/8214
Russian translation (actual version)
Merge pull request #1170 from abbat/russian-translation

Russian translation (actual version)
FIX: Recovering a deleted post was not updating a topic's statistics
And add tests for empty string & null
FIX: Don't allow a user to stage a post while another is being staged.
Use FuleUtils.mkdir_p for cleaner code
Subtle changes to border colors
FIX: Font Awesome upgrade changed padding on (+) Reply as New Topic
Check for updates: edge cases when the message on the dashboard doesn't make sense.
Fix order of View table in exporter
Refactor temp directory methods into helper module
Merge pull request #1165 from novemberkilo/feature/log-trust-level-boosts

Log all changes of user trust level by an admin
Merge pull request #1127 from rylnd/refactor_tmp_directory

Refactor #tmp_directory into helper module
Add a BIG WARNING to queue_jobs site setting
Add `deleted_by` to `Trashable` tables
Don't allow editing of title and category of an archived topic