Add `/lotto shift leave` command

[levb]

Needs to "do the right" thing when removing a user from a started shift. It should error on removing from a finished shift.

[FedjaVitko]

I would love to work on this ticket, just need a little bit of time to wrap my head around what the plugin does.
UPDATE:
Should be able to finish it today.

@levb what do you mean exactly by the "right thing"?
open =>
remove the users from the shift,
tell all other users in that shift that users have been removed,
notify the removed users
started =>
remove the users from the shift,
tell all other users in that shift that users have been removed,
notify the removed users
finished =>
don't remove the users,
error out

[jfrerich]

Good to see you again, @TheoVitkovskiy! I've added myself as a reviewer on your PR and will provide feedback this week!