Push functionality

[Norfeldt]

Push functionality

As a user I would like to receive a push message when a new message
is added to a room that I have participated in.

Accepttest

1. When i write a message inside a room, then i am asked if i
   want to have notifications from that room
2. Every time someone writes a message in the room, a push
   message is sent to me
3. When the user presses a push message, he or she should be
   taken directly to the room/message (using deep links).

[Norfeldt]

Seems like firebase linking is going to be shutdown