Only run a single repo_sync job at a time

It's currently possible trigger multiple calls to `/repo_syncs`
from the UI, by clicking the refresh link multiple times, or
reloading the page near the start of a sync.

This change ensures that `/repo_syncs` will not enqueue multiple
synchronization jobs at a time.
Fix Security Documentation

Document that we store user's GitHub token
during the "refreshing repos" phase.

Fix documentation to change that
we do not store the token
when enabling repos.

https://trello.com/c/qnFa7l7A/404-fix-security-documentation
Add tasks to remove duplicate data

Add repo:cleanup_orphans task

Add repo:cleanup_duplicate_github_ids task