Clean the access control code about mailu

[kaiyou]

After some discussion with pulser on IRC, it sounds like we have two choices: a full RBAC system (maybe based Flask-Principal for instance) or homemade decorators.

A proposed API would be a permissions module, exposing multiple decorators that would look like:

@permissions.global_admin
@permissions.domain_admin
@permissions.owner

Still, the permissions checks would require that the manipulated object is requested from database. So permissions checks would raise 404 errors, which sounds like an unrelated responsibility. Still no idea on how to fix this while still using decorators.

[kaiyou]

Permissions and confirmation forms were merged in a module named access. Current permissions API exposes:

# Enforces global administration privileges
@access.global_admin

# Model must be an Email subclass or Domain, the key must be a URI key
# Makes sure that the domain or the email's domain is managed by the current user
@access.domain_admin(model, key)

# Model must be a User or any model featuring a field named 'user' (Fetch and maybe some other user-specific resources later)
# Makes sure that the user belongs to a domain managed by the current user or is the current user itself
@access.owner(model, key)

[kaiyou]

I finished migrating all access-control features to the access module, removed the utils module and created a simple permissions audit script to avoid any surprise in the future.

[kaiyou]

And the audit script revealed a permission issue with domain deletion, which is great.

[kaiyou]

I just opened #55 for review before I merge anything into master due to the depth of some changes involved.