Add notifications stream

[juliushaertl]

From #34

• 🔔 Notifications: @mentions, new followers, people liking or reposting your posts or things you were mentioned in, you being tagged in pictures etc. ("Activity" as word for this is easy to misunderstand as it’s less specific)

[ArtificialOwl]

This will be postponed to 0.2 as we need to generate events/activity.

[ArtificialOwl]

We need some interaction from back-end to front-end to update on new post the UI, but we can also think about the in-browser web push notification for new notifications

[ArtificialOwl]

@juliushaertl would be nice to have a first version of the stream including at least the @mention for 0.1.1. For the like/repost/following you, we'll see that later

[StCyr]

would be fixed by #625