Add ability to subdivide a ticket into tasks

[tomek.piatek@…]

Some tickets can be pretty high level yet creating more tickets doesn't seem always right. It would be nice if one could add tasks (ie dependencies) to a ticket. One should not be able to resolve a ticket until all of the tasks it depends on have been completed. This would, in my opinion, allow software engineers to create a list of engineering tasks which need to be performed in order to resolve the ticket.

[cmlenz]

Duplicate of #886.