automatically grant access to enabled repos

[dcramer]

if X enables a repo and Y also has access, they need to automatically gain access in various situations

• on login
• on repo sync (via cache)
• background task? (we also want this to explicitly revoke access)

[dcramer]

we also need to automatically attempt access when they visit a repo

(e.g. I should be able to link everyone to https://zeus.ci/getsentry/zeus/builds/11/diff, and they should get prompted to auth and then gain access)

[dcramer]

Implemented grants on login, but this'll still need a bit more.

here's what im thinking:

1. we need to auto revoke access (via cron?)
2. we need to automatically attempt access when a '404' is hit on repository_base (this ones super tricky)

alternatively we could special case 'open source' repos, and not actually grant access, but also not require access

[jan-auer]

To revoke access, we could also listen to GitHub webhook events and then update immediately.