Add DM Browser/Desktop Notifications

[quinthar]

I think for us to drop Slack, we will need DMs to notify users. We already have this notification framework largely built out for web/mobile on the old codebase, so this should be a pretty straightforward exercise to port over to RN.

[marcaaron]

Just wanted to add some context / clarify some things about this issue.

Browser notifications on web are more or less ready for porting.

We will need to implement a snack bar of some sort in lieu of browser notifications for the native platforms (we don't need or want this for web/desktop clients)

If we want mobile device notifications i.e. integrating React Native with Airship's React Native module then that's maybe a separate issue that someone can look into.

[marcaaron]

Going to create a new issue for Airship.