Improve import experience by allowing gradual access

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When importing repositories from GitHub or BitBucket, the git part is the easiest and fastest step. If the project has a long history of collaborations on the platform, pulling all related data is subjected to API rate limits. This may lead to long hours of waiting.

We may have a few alternatives here to improve the overall experience:

1. surface when we are rate limited, so people know "who to blame"
2. just after we imported the repository, allow the user to see it (probably is a good idea to keep it 'read-only' so we don't break or make the import code too difficult, like trying to import a commit discussion, while the user just forced pushed and removed that comment)
3. keep all other interactions read-only, until you can finish the import completely

We currently have only one read-only flag on the project, and I believe it only affects the repository, not issues or merge requests, so we should probably add another one for those other places.

Discussion on hacker news: https://news.ycombinator.com/item?id=20768035