Dropbox support

[johanjanssens]

Files

Implement collection support for Dropbox HTTP API.
See: https://www.sitepoint.com/build-your-own-dropbox-client-with-the-dropbox-api/

Paper

Export a dropbox paper document to html or markdown to include it in the site
See: https://www.dropbox.com/developers/documentation/http/documentation#paper-docs-download

Resources:

• https://github.com/spatie/dropbox-api
• https://github.com/spatie/flysystem-dropbox
• https://developers.dropbox.com/paper-migration-guide