Notify participants when their ask to join the collaborative draft is accepted

Is your feature request related to a problem? Please describe.
Currently, after you ask to join a collaborative draft, you have to be accepted by the author. However, once accepted, you are not notified of the fact that you have been accepted.

Describe the solution you'd like
Notify participants when their ask to join the collaborative draft is accepted

Describe alternatives you've considered
Rethinking the collaborative draft workflow with letting everyone submit suggestions and just leaving the author to accept or decline them (kinda like in the change suggestions of google docs)

Funded by
If accepted, we can do the PR