New Event: EVENT_MANAGE_PROJECT_DELETE

Add new event: EVENT_MANAGE_PROJECT_DELETE

These events allow plugins to process or 'block' a project deletion,
as they occur prior to any version or project deletion.

This could be used to allow synchronisation with a 3rd party system, to deny
the deletion of specific projects.

Fixes #17783

Signed-off-by: Victor Boctor <victor@mantishub.net>