Add content guard

[bmclaughlin]

• Add content guard to check authentication before a collection can be downloaded.
• Create a migration to add content guard to existing Distributions
• Create a signal to add content guard to new Distributions

Issue: AAH-923

[newswangerd]

This should use get_or_create to initialize the object if it doesn't exist.

[newswangerd]

I believe the action on this view is already set to "download"

[newswangerd]

This is overriding the get_object function so that it always returns the content guard instance. What we need to do is make it so that get_object returns the collection that is being accessed.

However, looking through the access policy, it looks like the download action doesn't take any conditions, so we can probably get away without defining a get_object method on the viewset until we get to a point where we need to add conditions to the download function.

        {
            "action": ["download"],
            "principal": 'authenticated',
            "effect": "allow",
        },

[newswangerd]

For context, DRF access policy gets passed the view that is being accessed and can optionally use 'get_object' to load the object that is being accessed and check permissions on it. Here's one example where we do that: https://github.com/ansible/galaxy_ng/blob/master/galaxy_ng/app/access_control/access_policy.py#L75

[bmclaughlin]

@chouseknecht, @rochacbruno updated migration and signal.

[rochacbruno]

ci erroring on

django.core.exceptions.ImproperlyConfigured: Field name `content-guard` is not valid for model `AnsibleDistribution`.

[bmclaughlin]

/retest

[rochacbruno]

The Walrus := for the first time on our codebase :) 💯

[rochacbruno]

@bmclaughlin can you provide a testing workflow for this PR? here in comments, something like

1. Login to hub
2. Try to click on x, y z.
3. Verify that something happened

I want to help testing, but I checked out to this PR and I don't know what to check during my manual testing.

[bmclaughlin]

@rochacbruno The manual testing that I've done for this PR started as:

1. Make sure new migrations have been run
2. Verify our core distros in core_distribution have the same content_guard_id (empty prior to this PR)
3. Upload (or make api/create-test-collections), verify content_guard_id is populated again, same pulp_id

That is working as expected, I attempted to download an uploaded collection which now results in a 403 from the content-app. The content app is being re-enabled in #977, so I assume downloads should continue to work as they did prior to this PR.