Android Push Notifications

Support for sending push notifications (for now, only Android) to registered mobile users. Notifications are sent to users who have 'subscribed' to a model relevant to the data being sent. This PR depends on #1071, pairs with https://github.com/the-blue-alliance/the-blue-alliance-android/pull/237, and is a work in progress...
What you have now would allow you to query for Subscriptions where settings_json equals some exact JSON string, which isn't what you want. You want something along the lines of `subscribed_models = KeyProperty(repeated=True)`. This will allow you to query for all Subscriptions that are subscribed to `2014casj`, for example.

Also, properties default to being indexable.