Have configurable Firebase project

[blootsvoets]

As described in this Firebase resource: https://firebase.google.com/docs/projects/multiprojects, we could make the Google Firebase project configurable. This could happen either from ManagementPortal or from an app-server. That way each installation can use their own Firebase server for message sending.

This issue supersedes #584, opting for a configurable Firebase project instead of a multi-tenant one.

[yatharthranjan]

we can use this method — https://stackoverflow.com/questions/42117592/receive-push-notifications-on-one-android-app-from-two-firebase-projects
to have deployment-specific Firebase and app server (so our app server is not in increasing load, see RADAR-base/RADAR-Appserver#451). The FCM config in the app for different Firebase projects can be done using remote config from a central Firebase project.

• Select correct firebase project (using a central firebase remote config or radar-app-config) and setup app instance on login
• Use the base-url of the deployment (QR code) to get the appserver URL