Consider using instance id api to properly sync notification topics

[advayDev1]

Given that country changing is now allowed, the topic management code is fragile.

Instead of keeping track of which topics are subscribed to in the user db, instead go to the source of truth: https://developers.google.com/instance-id/reference/server#get_information_about_app_instances and add or remove topics based on that list.

[brunobowden]

Agreed. This part of the system definitely needs an overhaul.

[matthewblain]

@advayDev1 can you comment on this doc:
https://docs.google.com/document/d/1Itu46Qz43EVw2vasY0YvYDWydAlUyqMAEUgtRcROTGA/edit

[advayDev1]

I will take a look monday

[advayDev1]

Added comments for what i remember @matthewblain

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