Replace user levels with user permissions

[ptormene]

I believe it would be cleaner to define specific permissions and to create user groups sharing specific sets of permissions.
It would imply:

• defining all the required permissions
• removing the concept of user profile (that specifies the user level)
• changing all checks on user level with checks on single specific permissions
• before migrating, we should assign all level 2 users to a group sharing a set of permissions and all level 1 user sharing a different set of permissions