Simplify the PackageListing object (remove the acls2 attribute)
Add the has_acl method

This method returns a boolean specifying if a specific user has a specific ACL on a
specific package.

Add unit-test for this method
Prepare the is_admin method

This method will return whether a user is an admin or not. At the moment this method
only returns True but in the future it will take into account the groups in which the
logged in user is.
Add the image of the DB scheme