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
I'm inclined to say we should probably leave this test out as it's part of the test refactor PR and would cause conflicts. It's only one file though so I'm fine with it staying in.