Notifications Update

[FEATURE] Added canceling of pending notifications (that is ones that haven't yet been delivered).
[FEATURE] Added sales notifications to store subscribers.
[SYNTAX] Added -c flag to broadcasts (to cancel).
[ENHANCEMENT] Ending sales automatically cancels their notifications. I'm not entirely sure if we should send a "sale ended" message too or not.
[CODE] Re-wrote and added some notification methods.

--Left to do--
Add some config settings
Implement reports
Test AI notifications
The rest of the stuff that I wrote in Broadcasts and stuff update might as well be done in another branch or maybe the main one, I think.