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
The comments widget seems to be passing in the created timestamp without modifying it through `transformDate`. If it's not necessary to transform it before adding it we could do something like comments is doing here and in the other about widgets:

```html
<time datetime="${comment.created}"></time>
```