Optionally disable CiviCase email notifications to prevent duplicate emails

When using CiviCase, it's very useful to send out emails to activity assignees which prompt them to view/edit their activities.

The current behavior is to always lookup the assignees email and send a form message. This is a good default behavior because it's intuitive, it requires no configuration, and and it works with any case and any activity. However, it also coarse-grained: emails always go out at fixed point (right after editing), all emails use the same template, and all links point to the same "View/Edit" activity forms.

When activities involve untrained/poorly-trained users, it is important to customize the email content and activity form for different recipients and different activities. This fine-grained approach can be achieved with the "scheduled reminder" feature. (The downside, obviously, is that it requires more configuration effort.)

If a site administrator decides to use the fine-grained approach ("scheduled reminder"), then she should also disable the default, coarse-grained approach (CiviCase) to avoid duplicate emails. This is not currently possible.

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Proposal: Add a new tag to CiviCase XML which can enable/disable the default CiviCase emails. (This solution feels like the best balance of simplicity/configurability to me, but I'm sure other approaches would work, too.)