add email notification when just the form has changed in the database

[falkb]

A form change should trigger a ticket change notification, even if no change happened to the ticket (description/comment) wiki markup.

If several people are involved in the ticket form, changes to the form controls should notify the current owner, CC and reporter. If forms are embedded in tickets, users usually wonder if they get noticed about e.g. owner changes but not if the actual data changes.

Maybe this could also be implemented as Announcer plugin.

Note: this here is continued from a discussion in #10774.