Add Role and FieldPermission

Add migration, models, tests for Role and FieldPermission
Add default_role column to Instance model
Update model methods.

get_instance_permissions now optionally takes a model which,
if supplied, filters down permissions to those for a given model.
Add Authorizable mixin, rework mixin heirarchy

Add model mixin Authorizable to handle user authorization of
model level permissions.

Factor out UserTrackable class from Auditable to also be used
by Authorizable.

Add Authorizable mixin and test coverage to Plot. Make trees
authorizable and add tests. Update permissions in tests and
refactor setUp.