move file watcher functionality from sanitizer to bot

watchers are really pointless when sanitizing local commits, especially
as nobody actually has the relevant configuration.

as an additional boon, the bot can now automatically invite relevant
people.

Change-Id: I735056845d7f807904070e61efb54407dcec9fb9
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>