Update Dropbox oauth flow

Starting September 30th, 2021, the Dropbox OAuth flow will no longer return long-lived access tokens. It will instead return short-lived access tokens, and optionally return refresh tokens. Please be sure to review, test, and move to the new permission model before then.

For detailed instructions, please see:
- Dropbox OAuth Guide
- Blog post — Now Available: OAuth Scopes and Enhanced Permissions
- Blog post — Migrating App Permissions and Access Tokens