Add more advanced permissions to trees

[star4z]

Users should be able to change on their own trees:

• Universal permissions
• Permissions of individual users
• Permissions of a group

Users should also be able to create a group to make it easier to manage permissions for multiple users of the tree.

I think this should be implemented by a Permission object that contains foreign key references to a user or group and a tree, and also contains a permission level. Suggested permission levels: Read, Write, Blocked. Default for every tree would be universal blocked, with the exception of the creator.

Obviously, if Django has an option to do something like this already, use Django's option. (Django definitely contains logic for creating and storing groups, and some stuff on permissions. Note that the permissions may not be what we're looking for.)