Add X-Forwarded-To to allowed headers for email ingestion

When forwarding emails in Microsoft O365 (depending on the forwarding method) the forwarding target may be added to the X-Forwarded-To header.

If the customer wants to use the custom email feature, then this issue Custom Email: O365 forwarding doesn't add forwa... (#496396 - closed) might resolve any setup problems. But if the customer just wants to forward emails, those emails will be rejected.

Implementation

You can use this MR as a template Support X-Original-To headers for email ingestion (!149874 - merged)

1. Add header processing to lib/gitlab/email/receiver.rb
2. Add specs to spec/lib/gitlab/email/receiver_spec.rb
3. Add documentation to doc/administration/incoming_email.md