Slash Invite

- Fixed /whitelist list showing "null" for invited players who have never joined the server. - Ordered usernames by date, so now you will always be able to see the /invitelog of recently joined players instead of stale entries. - Added pagination to /invitelog. You can now page through five entries at time. - Added filters for /invitelog to search invite log by most recent, by who invited a player, and by who a player invited. - Added /uninvite to allow normal players to revoke an invitation. - Added config option cooldown-seconds to configure cooldown between /invite that hit the Mojang API. - Added config option undo-seconds to configure the length of the window in which /uninvite works. Warning: there is a migration for the invite log database in this update. A backup file (`invites.db.bak`) will be created beforehand, but you should keep your own backup just in case.

Initial public release v0.1.0. Hopefully not too broken.