rest: handle sharing message through notification and email

[DaanRosendal]

Messages are currently not being handled when a workflow is shared. This task might involve changes to the database depending on how message handling is implemented. E.g. there could be a new table for messages which saves memory in cases where a workflow is shared with multiple users at the same time, preventing duplicate storage.

Two actions are to be implemented:

1. Trigger notification
2. Send email