Gsr notifs2.0

[rchhaya]

GSR Notifications with the following functionality

• Notification comes up ~10 min prior and lasts until the start time of the GSR
• Notification has a short text version (generic) and longer text version (w/ room number and timestamp)
• Clicking notification opens the app but does NOT close the notification
• Booking multiple GSRs at the same time creates multiple notifications (achieved by modifying MainAct companion object)
• Booking and then canceling the GSR will remove the notification (achieved by modifying MainAct companion object)
• If the GSR is scheduled within 10 minutes of the start time, no notification is sent (since the notification would be sent immediately and it would cancel after the GSR time starts which doesn't really make sense)
• Lots of null checks and contains(key) checks to prevent crashes if something goes wrong