Folders UI

Initial PR to get a sanity check on the work done so far.

The following items have not been tackled yet:

- Changing the visibility of a folder
- Deleting folder(s) from the system
- Adding existing items to a folder
- Change the URL when switching between the content and folders tabs
- A large part of the tests
We have to cache this at the widget level. We need to pass the default visibility in when initialising the setpermissions widget. The setpermissions widget will pass in the update version when it's finished. We need to cache this so we can use that visibility setting when eventually creating the discussion. Given that the setpermissions update and the create are 2 distinct actions, there's not really another option that wouldn't significantly increase complexity.

This is also a pattern that we were already using in all of our other `create` widgets.