Admin Push Messages

[samhill303]

*copied from internal tracker
Old versions of the app used to be able to listen for events sent from Firebase cloud messaging. I'd like to add that back into the app. The messaging should only really do 2 things.

1. Tell the app to update the schedule. We may have made schedule changes on the sessionize server and we want the app to run the update now. This won't show anything to the user. It'll just tell the app to do something.

2. Admin message. Just a text string that will show up in the app. Just in case there's some announcement the conference organizer would like to send out.