Support multiple email domains when creating issues/comments from email

With two different domains one user effectively has two different email addresses - user@domain1 and user@domain2.
Email handlers should be able recognise that it is the same user and add comment / create issue as appropriate user.

Currently if email to JIRA is sent from user@domain2 JIRA ignores the email.

Studio has already provided this partially by overriding one of JIRA's class. Pleaes see JST-3171. Currently this can only be modified by a sysadmin.

This fix does not support assigning issue from email if the assignee's email address is using an alias. Please see JST-5079 to track its progress.

When JIRA implements this improvement, it would be appreciated to keep this backward compatible with Studio.