Revoke reviewer invitation

Similar to #263 (closed) and #264 (closed) in that it needs to happen both automatically and manually.

Revoking the invitation manually should be done in the reviewer management page.
Automatic revoking should be a cron job. In combination with #264 (closed), it would send a reminder after x days, and revoke after x + y days.

All pending invitations should be auto-revoked when a decision is made.

This should not delete the invitation, but keep the info (maybe with a status on the team member?)