Custom domains: track validation process (#9428)
Started putting this code in -ext, but since it doesn't depend on those models, I'm putting it here. And creating a new app to put all domain related code (can't name it cdn, because we already have that app name, but we can rename it once we have migrated all from that app).
This implements https://github.com/readthedocs/readthedocs-ext/issues/490, only the deactivation part, we don't delete domains.
Basically, the user has 30 days to complete the setup, or we will strop retrying, but they can restart the process by saving the domain if they run out of time.
### How to deploy this change
- Run 0091 before releasing the webs
- Run 0092 after releasing the webs
