Enable cancelling of inflight Invitations

The sync bot should be able to cancel in-flight invitations in the case where a GitHub User has joined the Organization Member group after getting access to an individual team.

This will help resolve an issue some users are facing- where they are never added to GitHub Teams due to an existing Org Member Invitation being present.

Start at the sync bot's handling of individual-team access and Organization Member membership. Trace how existing Organization Member Invitations are detected and where invitation cancellation belongs. Done means an in-flight invitation is cancelled after the user joins the Organization Member group, allowing the user to be added to GitHub Teams.

索引モデルが issue の本文から書いたものです。