Fix: let mailgun_api_key also support their "HTTP webhook signing key" (PR #8091)

Mailgun now have a new API key format for webhooks which wasn’t matching the setting regex.

Follow on to https://meta.discourse.org/t/handling-bouncing-e-mails/45343/131?u=dannii

GitHub

You’ve signed the CLA, curiousdannii. Thank you! This pull request is ready for review.

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/handling-bouncing-e-mails/45343/132

Seems good to me, thanks!