Make it possible to handle both Mailgun's temporary and permanent failure hooks.

Problem

This is an infradev item because we had a week-long incident where e-mail notifications were delayed. We think the large number of Mailgun failures (both temporary and permanent) might have quietly induced a large build-up of retries on Mailgun's side. Usually Mailgun will add a permanent suppression to avoid affecting our sending reputation, but in this case we saw large number of temporary failures that should have caused us to stop sending notifications

Proposal

• Make it possible to handle both Mailgun's temporary and permanent failure hooks.
• Related code: https://gitlab.com/gitlab-org/gitlab/blob/9c8a128ea056ee9170c7a32ad28a65900ec873aa/app/services/members/mailgun/process_webhook_service.rb

Engineering breakdown

Things we'll need to do here:

1. Redirect / alias /-/members/mailgun/permanent_failures to a more generic /-/mailgun/permanent_failures
2. Add /-/mailgun/temporary_failures or we could just make this one endpoint to handle both if payload has data to identify if it's a temporary or permanent failure
3. For these new endpoints, detect "invite email" failures and call Members::Mailgun::ProcessWebhookService on those. For the other failures, we will handle them in the next issue