Bug 1005404 - Add ability to create task within tool

This is my first attempt to implement the features requested by bug 1005404. This includes:
- Changes to the model to support the new metadata for tasks
- A form that an admin can use to create a new task
- A form that an admin can use to edit an existing task
- A listing that an admin can see with all tasks, from whence they can choose one to edit and also can click a button to be taken to the "new task" form

Everything seems to be functional and working, and matches the spec in the bug, including limiting access to admins. The form *does not* look good. It needs css work, and that is far from my forte. I am hoping that someone currently on the project can help with that, or perhaps we can recruit someone to help with it.

As for the django code, I am definitely uncertain about a lot of it, so I am hoping for feedback from @Osmose. As I said, I believe that it all works, but I'm not sure if I did a lot of it correctly, including how I dealt with assets and the .less file.
You should make `AvailableTasksFilterSet` a subclass of this.