wip: big refactoring

Moving files.
wip: Big refacto

The admin application is now a true extension.
wip: Big refacto

Top menu is back.
wip: Big refactoring.
wip: Big refactoring

Limits extension OK.
wip: Big refactoring

autoreply extension OK.
wip: big refactoring

stats extension OK.
wip: big refactoring

Webmail still needs work.
wip: big refactoring

* sieve extension still needs work
* auth application removed
wip: big refactoring

Webmail template tags.
wip: big refactoring

amavis extension still needs work.
wip: big refactoring

old userprefs application removed.
wip: big refactoring
Useless import removed.
Support only integers for this option.

fixes #478
Mailboxes manipulation fixed.

fixes #479
The gunicorn command line was wrong.
Missing import added.

see #459
Don't forget aliases :p

see #459
Handle mailbox operations in background.

see #482