Improve the postfixadmin migration code:

    - Assign right permissions to domain administrators and
      superadministrator.
    - Correctly prepend the password schema to all admins.
    - Update model classes with the correct varchar length and
      other useful relations.
Merge pull request #609 from carragom/1.1.x

Improve the postfixadmin migration code
Last fixes.

see #609
Documentation update.

see #609
Merge branch '1.1.x'
Amavis configuration update.

see #608
Merge branch '1.1.x'

Conflicts:
	doc/getting_started/plugins.rst
Amavis configuration update.
Fix attempt.

see #610
Merge branch '1.1.x'
End of fix :p

see #610
Merge branch '1.1.x'

Conflicts:
	modoboa/extensions/admin/forms/account.py
Merge branch 'master' into feature-responsive

Conflicts:
	modoboa/extensions/admin/static/admin/js/admin.js
	modoboa/extensions/admin/templates/admin/identities.html
	modoboa/extensions/stats/static/stats/js/stats.js
	modoboa/extensions/stats/templates/stats/index.html