Dropbox code needs to be updated to use refresh tokens

[DougReeder]

Dropbox no longer issues "long-lived tokens", only tokens that last about four hours: https://dropbox.tech/developers/migrating-app-permissions-and-access-tokens#updating-access-token-type Users must click a link every four hours to re-authorize the app.

To regain the original user experience (what Dropbox calls "offline access"), the Dropbox code must be updated to use "Refresh tokens" to renew the access token.