Start using a folder/document picker rather than the "ui-tree"

Essentials has a few configuration screens where you need to select a content folder. This is currently implemented using (an older version of) ui-tree. This is inappropriate, and we should start using a dedicated folder/document picker instead, optimally provided by the Hippo Theme.

Current issues include:

• Tree uses a hardcoded max depth of 3
• Tree styling suggests that it is draggable