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 fine with having 2 pushes here but we could combine them into one and put the docs on top.