mail_drop_target: mail_drop_target.disable_notify should be True by default

[glasl]

Is your feature request related to a problem?
By default, when adding an e-mail via drag&drop a notification is sent to all followers.
This is problematic e.g. in the CRM module, for example when:

1. The customer is already follower.
   They didn't reply to the e-mail sent out of Odoo.
   You drop the e-mail into the odoo mail thread.
   The customer receives an irritating duplicate of his own e-mail.
2. Worst case: You drop in an e-mail containing confidential information related to the conversation.
   Odoo sends the whole e-mail to the customer as notification, containing confidential information including sender and receiver.

This is a potentially dangerous default and should only be set in the special case it is required.

Describe the solution you'd like
Set the setting mail_drop_target.disable_notify as True by default

Describe alternatives you've considered
An alternative would be to send the e-mail only to internal odoo accounts that are followers.