Add new command: Migrate issue to different repository

[Makman2]

A new command like

corobo migrate coala/coala#1234 to coala/coala-bears

should be supported. It would

1. copy discussions (as citations, mentioning who commented that (and maybe when))
2. reference the original issue (saying Migrated issue from ... (by <author>).)
3. Write in the original issue Migrated to ... (by <author>).
4. apply labels from the original issue
5. mark the original issue as invalid
6. and finally close the original issue.

Only maintainers should be able to perform this command.