Service Desk should add CC'ed users to the issue

Summary

When incoming email addresses are cc'ed on issues coming to the service desk, they should also get a copy of responses to the issue.

Steps to reproduce

Send an email to the Service desk and CC a friend. Respond to the issue. Note how friend never gets a copy.

What is the current bug behavior?

The cc'ed email address gets no issue updates.

What is the expected correct behavior?

The cc'ed email address should get cc'ed on all responses. If a cc'ed user were to respond, their response would be added to the correct issue. The issue comment should also say which email the comment came from.

Results of GitLab environment info

GitLab Enterprise Edition 10.3.3-ee