[#2401] Add management command to set Project.last_update
[#2401] Add management command to set Project.last_update

Since the bug where Project.last_update may be None after deleting an update has been around for some time, create a management command to tidy thing up.

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr