Model / settings for allow-listing

(See #853 (closed) for context)

This MR adds:

• Two dedicated admin settings to control if:
  • Allow-listing is enabled
  • The list of domains on the allow-list is public or not
• A new allowed field in the Domain table, to control wether or not the domain is on the allow-list. We default to Null so mods of pods switching to allow-listing can quickly find out the list of domains they need to check , and batch allow/disallow those
• Display of the allowed value for each domain in Django's admin
• The corresponding allowed field for Domain resources in the API
• Support for updating this field when you're a moderator

I know it may seem a lot, but the MR is actually quite small, and each commit match precisely one of the items listed above. I recommend reading the changes commit-by-commit ;)

Let me know if you have any question!