Add X-Auto-Response-Suppress header to outgoing messages (#6492)
Add X-Auto-Response-Suppress header to outgoing messages (#6492)

This is a header used to suggest mail servers not to respond to a message
with an auto-responder:

 https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcmail/ced68690-498a-4567-9d14-5c01f974d8b1

This header is also used by github and gitlab in outgoing messages.

Fixes #6484
cez81/gitea,lafriks/gitea,lafriks/gitea,sapk-fork/gitea,cez81/gitea,cez81/gitea,kubatyszko/gitea,kubatyszko/gitea,go-gitea/gitea,lafriks/gitea,sapk-fork/gitea,typeless/gitea,sapk-fork/gitea,vdbt/gitea,typeless/gitea,go-gitea/gitea,typeless/gitea,typeless/gitea,vCaesar/gitea,vdbt/gitea,sapk-fork/gitea,go-gitea/gitea,kubatyszko/gitea,vCaesar/gitea