Add occ command for getting User ID(s) for a given email addr…

[stellarpower]

Summary

Adds an occ command for getting the user ID(s) from an email address. Uses the user manager to lookup the email address, and returns a list of users if they have this address listed.

Uploading so that the community can benefit from contributions, but please do feel free to pull into a feature branch and rework or modify heavily and use as a springboard. This solved a particular usecase I had so figured may be useful.

TODO

• I'm by no means a PHP developer, so there may be significant defects.
• Don't have the ability to test against current master; slight modifications and it appears to work on my installed server, running NC 21, but would need to be tested properly against master code.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)