Regroup all queries in one place.

see #589
Fixing demo fixtures.
Merge branch '1.1.x'
Do not override the default history object.

fix #588
Merge branch '1.1.x'
Few fixes in the postgres backend.

see #589
Adding some comments.

see #589
Merge branch '1.1.x' into refactoring/amavis_queries
Merge branch '1.1.x'
Merge branch 'master' into feature/version_checker

Conflicts:
	modoboa/extensions/amavis/general_callbacks.py
	modoboa/extensions/amavis/views.py