Audit on-site notifications

We want to audit all the on-site notifications to confirm their copy makes sense and they are useful. Basically, we want to confirm if a user that reads the notification and clicks on the link to "solve the problem" has all the context required on the linked page to be able to solve it without requiring jumping into the Read the Docs documentation.

A good example of a not useful notification is the notification for a failure webhook attached:

The project {{ project.name }} doesn't have a valid webhook set up, commits won't trigger new builds for this project. See the project integrations for more information.

(from https://github.com/readthedocs/readthedocs.org/blob/b68a92106b5926c7e708f77aa3d04e2f30b0a24b/readthedocs/oauth/notifications.py#L44-L48)

that links to the Admin -> Integrations page that has no context. Example: https://readthedocs.com/dashboard/readthedocs-ops/integrations/

Original card at: https://trello.com/c/tY0Mi8v4/771-audit-all-trial-subscription-emails-notifications-to-ensure-they-work-and-are-useful

Start with readthedocs/oauth/notifications.py, especially lines 44-48, and inspect the linked Admin → Integrations page. Audit the on-site notifications and their linked pages for useful context; done means each notification's copy and destination let users solve the stated problem without consulting Read the Docs documentation.