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
And add tests for empty string & null
Use FuleUtils.mkdir_p for cleaner code
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
French translation of user_replied/posted subject

This is a follow-up of the commit 013ad0fdd
rake autospec should handle css file watching and reloading, guard is a bit pear shape at the moment
The flattening part 2, cleaned up styling so it is more consistent with the removal of border yesterday
quote style now matches block quote style
glyphs on the user stream, rename responses to replies
TODO comment
Correct caption of 'Make Visible' button

The variable 'visible' is also used for toggling the admin menu visibility,
so it is always true when the visibility toggle is shown. Hence, it's caption
is always 'Make Invisible', even when the topic already is invisible.