Add migration for Role and FieldPermission
Add models and tests for roles and field permissions
Add migration for default_role column in Instance
Add default_role 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