ensure product/category pairs never have NULL categories

[Pomax]

Closes #7814
Reverts #7776

This change updates the ProductPageCategory model (which represents a (product <-> category) many-to-many relation) so that if a category is deleted, any ProductPageCategory that uses that category is deleted, too, with a migration that explicitly deletes all currently extant ProductPageCategory instances with a category set to None (there are 171 entries in the production database for this at the time of writing this PR).

Verifying the problem

• check out main
• copy the production database using inv copy-prod-db
• docker-compose up
• try to load http://localhost:8000/en/privacynotincluded/roku-streaming-sticks/
  • this should work fine
• log into the CMS
• go to the "Buyers Guide Product Categories" snippets section
• delete the "entertainment" category
• try to load http://localhost:8000/en/privacynotincluded/roku-streaming-sticks/
  • this will be a server error 500
• in the CMS, search for "roku streaming sticks" and edit the (en) version
  • the categories should show:
• delete the "empty" category from that list
• publish the product page
• try to load http://localhost:8000/en/privacynotincluded/roku-streaming-sticks/
  • the page should load fine again.
• try to load http://localhost:8000/en/privacynotincluded/nintendo-switch/
  • this should still be a server error 500. This will be fixed in the solution testing
• go to the edit page for the (en) nintendo switch
  • observe the "emtpy" category, but do not change/publish anything.

Testing the solution

• docker-compose down
• check out this branch
• run inv migrate
• docker-compose up
• load http://localhost:8000/en/privacynotincluded/nintendo-switch/
  • this should not have any errors
• go to the edit page for the (en) nintendo switch
  • there should not be an "empty" category in the edit view.

[danielfmiranda]

Hi Pomax! Just tested this out following the instructions provided and can confirm that it works great! Approving 👍