I propose to add the capability to send a mail template to speakers of any subset of event. A clear use case is that we want to send instructions to all demo stakeholders; and a different instruction set to submittors of lecture.

This also opens the door for additional batch editing options, directly from the "All events". For example: change tracks; types; reassign state.

The GUI is not explicitly clear that the bulk-mail is not subject to pagination; i.e., If you have 150 events matching the criteria, only 100 are displayed per page, but the "Send" mail will send to the 150 events' stakeholders. That's why the confirm dialog box specifies the number of events which will be notified.

#568 is a prequisite (but can be done without if it's important to someone).

I want to deliver this after #555 but I can remove the dependency.