Add CinderPolicy model and sync_cinder_policies management command

[KevinMind]

Fixes: mozilla/addons#9434

Description

1. update CinderPolicy to make uuid unique and add parent field for nesting. On a separate commit to be cherry pickable if we want to push early/separately

2. add a task/command to sync cinder policies to our database.

Context

Since we want to be able to trigger this either via manage.py or via cron a task seemed appropriate. There are many opinions about when to use tasks so happy to hear feedback.

Testing

1. you need a CINDER_API_TOKEN (talk to @eviljeff). set it in settings_local
2. make shell
3. ./manage.py sync_cinder_policies
4. Check your local database and expect CinderPolicy records to exists. Policies are related via nested_policies as child policies.

[diox]

Oh, while you're at it, to help debugging, consider adding a simple __str__ to CinderPolicy like this one:

    def __str__(self):
        return self.name

[eviljeff]

I don't have anything to add beyond Mat's review. My merging of another PR has caused a merge conflict in this patch though.

[KevinMind]

There is a conflicting migration as well. Recreating mine.

[KevinMind]

How did this not trigger a CI error? @diox any ideas? I suppose our linter isn't actually tracking dependencies? Should that be the case? Feels wrong.

[diox]

You didn't have a test covering the management command, so that didn't break CI