Rescan for github contributors at startup

During an outage, the bot server will miss github webhook calls and therefore could end up out of sync if a melded user became a new contributor. Later contributions would fix it, but wouldn't if they only had one contribution during the outage.

Start at the bot startup path and the existing GitHub webhook handling, then trace how linked users and contributor roles are updated. Determine how a startup rescan should find contributors missed during an outage and verify that a user with only one contribution is synchronized.

Written by the indexing model from the issue text.