Implement Firebase In-App Messaging

[shockbytes]

Overview

Implement Firebase In-App Messaging for sending context-specific announcements to the users. Previously, in Dante, this was done with hardcoded announcements. Since announcements should be made without a new release, Firebase In-App messaging should be used.

Implementation

Use the standard package for this: https://pub.dev/packages/firebase_in_app_messaging

[lockieRichter]

@shockbytes I've tried to do the "Test on device" steps as described in the getting started document, but the button is disabled for me. I assume I'm missing a permission or something. Do you want to enable the permission for me or just do this one yourself?