labhub.py: add migrate_issue plugin

[aabhaas-vaish]

Introduce migrate_issue plugin that adds ability to migrate an issue from a source repo to target repo, both owned by the org.

Issue title, issue description and all comments are copied with a few additional details appended to the the description and comments. Source issue is referenced in the target issue and is closed after the migration is complete.

Closes https://github.com/coala/corobo/issues/518

Reviewers Checklist

• [ ] Appropriate logging is done.
• [ ] Appropriate error responses.
• [ ] Handle every possible exception.
• [ ] Make sure there is a docstring in the command functions. Hint: Lookout for botcmd and re_botcmd decorators.
• [ ] See that 100% coverage is there.
• [ ] See to it that mocking is not done where it is not necessary.