Implement #3243 New Issue by email
So we extend Gitlab::Email::Receiver for this new behaviour,
however we might want to split it into another class for better
testing it.
Another issue is that, currently it's using this to parse project
identifier:
Gitlab::IncomingEmail.key_from_address
Which is using:
Gitlab.config.incoming_email.address
for the receiver name. This is probably `reply` because it's used
for replying to a specific issue. We might want to introduce another
config for this, or just use `reply` instead of `incoming`.
I'll prefer to introduce a new config for this, or just change
`reply` to `incoming` because it would make sense for replying to
there, too.
The email template used in tests were copied and modified from:
`emails/valid_reply.eml` which I hope is ok.
/cc @DouweM #3243
See merge request !3363
4a2320a7b0