GitHub: Migrations support next steps

[amitsaha]

Currently the github.createUserMigration command will attempt to create migrations as follows:

1. Get all the user owned repos and create a user migration (this option respects all the repo options such as membership type, ignore forks, starred)
2. Wait for the migration to be ready
3. Download the archive
4. Get all the organizations the user is an admin of, a.k.a an owner of
5. For each organization in (4), list all the repositories in those organizations and create a migration, wait for it to be ready and download the archive.

This behavior should be improved in the following ways:

1. Allow user to filter org repo types
2. Improve implementation in various ways
3. Implement some sort of a way to summarize the backup