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're using either 'bijgewerkt', 'gewijzigd' or 'bewerkt'. I'm not biased towards any of them but we should probably settle on one at some point.