Mark a user as "being synced"

[andrew]

Whenever we start downloading repos and permissions for a user they should be marked as "syncing" and a spinner shown on the Repository monitoring settings page, which can then be hidden once it's done, with a timestamp of last sync time, update to the active page to be done with pusher.

• add new User field currently_syncing boolean
• add new User field last_synced_at datetime
• toggle currently_syncing in #update_repo_permissions_async
• update last_synced_at after #update_repo_permissions_async finishes
• show spinner on /dashboard if currently_syncing is true otherwise show last_synced_at
• sync users every 24 hours
• "Sync now" button on /dashboard
• use message_bus to update /dashboard when #update_repo_permissions_async is complete

[andrew]

Moving this to the Backlog as we'd still like to implement it but can't see that happening in the near future.