Out-Of-Office now support ManageSieve backend

Two backends to store the Sieve script: file (current and default one) and ManageSieve. The latter requires sievelib to work.

It is now multi-tenancy ready by reading patched samba configuration key `auth:usernames are emails`.

Other fixes in OOF:
- Fix the SIEVE script when it was enabled/disabled.
- Keep user script as include if other active Sieve script was previously set.