Generalize Mailgun webhook endpoints
This used to only process member invite email failures but we want to use this webhook to track other email delivery failures too
Changes /-/members/mailgun/permanent_failures
to /-/mailgun/webhooks
so we can use the endpoint for other types of webhooks. The plan is to use this for tracking delivery failures for notification emails.
Related to #359113 (closed)
Edited by Heinrich Lee Yu