Feature: send email notification to share recipients

[ASerbinski]

I would like to be able to set up a flow that sends email notifications to all email sharing recipients with access to an affected file.

Our use case is that we have a group folder (https://apps.nextcloud.com/apps/groupfolders) containing a subdirectory for each of our clients used for bi-directional data sharing with the client, each of which is shared to the applicable client by email. When we upload files to those client directories, we would like the applicable client to be sent an email notifying them of the data.

Preferably, this could be set up at the administrative level, but it could be worked around at the user level as well.

[delacosta456]

this are related #28