Initialize Dropbox authorization

From the OAuth2 authorization list a page action is available to create a new Dropbox OAuth2 authorization using a authorization code grant flow.

This creates an OAuth2 authorization record with the basic setup for Dropbox.
The client id and secret are still required and can be found in Dropbox in the app settings (App Key and App Secret).

If an Oauth2 authorization record already exists with code DROPBOX, a dialog will pop up with the question if an additional record should be created.