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 (closed)