Add option to receive or not private note notification

Is your feature request related to a problem? Please describe.
On one side, Admins can add notes on the proposal for the other admins and valuators but they don't really know that there is a note until they check its dedicated section. Only a counter in the proposal admin index allows admins to know if a proposal received 1 or several private notes, but as an admin, from the index there is no indicator of how recent they are.

On the other, admins complains of the large number of emails and notifications received for each private note added.

Describe the solution you'd like
The solution we propose is to add on Participant settings - Notifications settings an option to receive or no the emails & notifications. 

Additional context

Does this issue could impact on users private data?
No

Funded by
Could be funded by Open Source Politics