Customers have to create a Dropbox app and set the secret key in the superadmin before this can be used (Pixfizz needs to create documentation on setting up the app!).
To use it, add dropbox to the sources="..." parameter on <px-(multi)-image-upload> in the CMS, and also add dropbox-app-key="{{ website.dropbox_app_key | escape }}" parameter to the same element.
