Support multiple repositories

[abhidg]

Currently ticket-dispatcher supports a single email domain, dispatching to a single repository. Allow a mapping environment variable that can enable posting to multiple repositories, e.g.

TICKET_DISPATCHER_DOMAIN_MAPPING="foo.example.com=org/foo bar.example.com=org/bar"

If SES is setup to deliver emails from both domains, then ticket-dispatcher should be able read To: or Cc: and deliver emails to the appropriate issue tracker.

[abhidg]

Simplified version: emails sent to 123+topsecret@issues.example.com will be sent to the same organisation as GITHUB_PROJECT with repository topsecret.