Enable direct storage provider config with Dropbox

[markmhendrickson]

This epic entails the exploration of a direct way for users to authorize with their Dropbox accounts a Gaia hub that supports many-to-many associations between Blockstack IDs and storage providers.

The goal here is to empower users who want to store their app data in consumer providers like Dropbox without having to host their own hubs. In this scenario, hubs would serve as trusted middleware for any number of users who wanted to store their data in any number of storage providers, and as such, would no longer dictate a default storage provider necessarily.

Dropbox is considered a good first consumer storage provider for this exploration due to its general popularity, robust API, and ability to authorize app access to a single user directory (avoiding the security issue of authorizing a user's entire drive and exposing that vulnerability to a break in trust with the hub host).

If we find that Dropbox support is successful, we could then expand such support for other consumer providers with APIs such as Google Drive.

This work will entail modifications at various levels of our tech stack. Our PBC Gaia hub will need to be modified to support the simultaneous configuration of various storage providers for various users. And the browser will need to present and broker the user's decision to authorize their Dropbox account via oAuth 2.0 with that hub.

Note that this experiment will likely not be introduced as part of the onboarding flow but rather a secondary setting in the Browser UI per Blockstack ID for those users who want to migrate from the the default storage provider as established by PBC's hub or any other they configured previously.