[notifications-server] Init notifications-server for Cashtab push notifications on android and web

Summary:
Ref D20189

Here we init the actual server with its CI tests and build

We will implement the notifications in Cashtab in a follow-up diff

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D20198