Setup basic push notification functionality

[shuesken]

A minimal working example would be a user receiving a notification whenever someone posts a note within 10km of a note the user posted. This will require our server to know what nostr pubkey is linked to what device in the form of an ExpoPushToken and our server to do some calculations upon receiving new notes to identity recipients. It should cover all the the infrastructure bits for setting up a more fine-grained push notification structure later with replies and defined areas of interest.