Encrypt Dropbox OAuth token & account details

[robertknight]

The Dropbox OAuth token is currently stored unencrypted in local storage after the user logs in. It ought to be encrypted along with the rest of the user's data.

This is a step towards allowing temporary sign in on shared systems in future, although I think that use case would be better served by allowing the user to easily send only the credentials they need from a device they trust (eg. phone/tablet) to the shared system.