Add appsMeta with pubKey to profile (for storageConnected apps)

[friedger]

This PR

• add appsMeta to the user profile (next to apps) that contains
  • the public key for the user for the app (public_key) and
  • the gaia hub url as in apps (storage)

Required for https://github.com/blockstack/blockstack.js/issues/458

The profile looks

[hstove]

Thanks, Friedger! I like that the benefit of this is pretty high, given not too much complexity. One thing I'll add is that we definitely need tests before merging this in. We already have tests around the publish_data scope, so you can probably follow those patterns to test this.

I'll bring this up in our next meeting around the user experience team.

[CLAassistant]

All committers have signed the CLA.