Add some improvement on the rendering of the notification

[r0ms]

Hey ! As promise, this is a slightly improved notification for a slack room.
Here is what you will get on desktop :

On mobile it give this :

Hope I it will help ;)

[r0ms]

Just one thing, users will have to set the name of the bot and avatar in the slack administration. It's not a big deal but it can be interesting to give them this information ;)