Fix home feed to update in real-time

[adizcode]

Description

The home tab only fetches the note feed when its viewmodel is initialized (once until the app is restarted)

Solution

1. First of all, instead of processing information at the Android-side, Firebase Cloud Functions should be used to keep track of all public notes in a separate collection.
2. Then rewrite home tab and its viewmodel to fetch the notes feed collection ordered by timestamp.