Add syncing statuses to GithubRepo

[joshsmith]

Problem

GithubRepo needs new columns like:

• sync_state
  • unsynced (default)
  • fetching_pull_requests
  • errored_fetching_pull_requests
  • syncing_pull_requests
  • errored_syncing_pull_requests
  • fetching_issues
  • errored_fetching_issues
  • syncing_issues
  • errored_syncing_issues
  • fetching_comments
  • errored_fetching_comments
  • syncing_comments
  • errored_syncing_comments
  • receiving_webhooks (final state, indicating it’s now just listening)
  • disconnected?
• syncing_pull_requests_count
• syncing_issues_count
• syncing_comments_count

For the sync_state, we'll want to add changeset validation on inclusion of those columns.

The question around the disconnected state surrounds whether we want to outright remove a GithubRepo from a project, or simply set it to disconnected (so the user can resume syncing without losing the existing data).

This is probably a bigger question that subsumes this issue and largely lies outside of it. My vote is to not add it for now, but to potentially have this discussion in another issue. Thoughts @begedin and others?

[joshsmith]

I'm not sure now whether this should be on GithubRepo or ProjectGithubRepo. The former may make more sense since GithubRepo is actually the atom of operation for Github.Sync syncing.