GitHub/Bitbucket/etc. importers should provider better feedback about import status

After an importer launches, it's not really clear to the user the progress of the import: whether it failed, whether it's still importing repositories, pull requests, etc.

Ideally, we would be able to show in real-time when the user sees "Importing in progress":

1. Progress of the git fetch (at the moment, Gitaly silences this, so we have no idea)
2. % complete of pull requests
3. % complete of issues
4. % complete of milestones
5. Failures, if any