Allow any combination of channels in scanner query rules

[chrstinalin]

Closes mozilla/addons#16299

Description

Testing

(Optional) Enable the enterprise-channel waffle switch. Create enterprise add-ons.

1. Check out master.

2. Navigate to admin/models/scanners/scannerqueryrule/add/. Create some rules with and without run on specific channels.

3. Check out the PR branch. Run the migration.

4. Existing rules should behave and display as expected.

5. Navigate back to admin/models/scanners/scannerqueryrule/add/.

6. Create a new rule: toggle Run on specific channels to true. Select some subset of channels. The rule's results should only show the expected channels.

7. Create a new rule, leaving Run on specific channels set as false. By default, the rule runs on all channels.

8. Create a new rule with specific channels set. Edit the rule and untoggle Run on specific channels. Should default back to running on all channels.

9. List view should correctly show which channels are being filtered to, if set.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.