Initial work about SA training from quarantine content.

see #622
Better settings to control manual learning.

see #622
Documentation.

see #622
Allow per-domain learning.

see #622
Documentation.

see #622
Translation update.
Don't check sub levels for super admins.

see #622
Possibility to filter clean messages.

see #622
Fix attempt for #630.
Better fix.

see #630
Remove incorrect reference to request.

see #630
Check group instead of mailbox.

see #630
request.user...

see #630
Fix old response format.

see #622
Few fixes.

see #622
French translation fixes.
Do not double escape search patterns.

see #622
Load relay domain list if extension is activated.

see #631
Convert to string if necessary.
Process statistics for relay domains too.

fix #631