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
There isn't at the moment. It's a larger piece of work that also involves other role changes (note that the comment was already here before). It's an item on our backlog, but it hasn't come to the top as a priority yet.