Implement service for blockstackAuth

[benoror]

blockstackAuth object currently lives in src/app.js

• [ ] Should it be a separate service/singleton fore re-use?
• [ ] Use state-management library like Redux or start using Hooks?

Related to:

• https://github.com/blockstack/blockstack.js/pull/542
• https://forum.blockstack.org/t/proposal-giving-control-of-session-state-to-app-developers/6006/20?u=larry