Update Dropbox oauth flow

[Sneezry]

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

[mymindstorm]

This should be almost identical to the OneDrive implementation except for PKCE.

EDIT: looks like we need to update the auth for OneDrive too, there is a message to upgrade to a newer auth flow that uses PKCE.