oauth logic

Logic for the Oauth popup

• App is opened with "?action=challenge&host=xxx&id=xxx&challenge=xxx"
• Popup an "Authorize With" modal, that shows the "host" url
• When user clicks Authorize:
  1. GET relay /signer/{challenge} (returns an object with "sig" field)
  2. Open user's default browser: https://auth.sphinx.chat/oauth_verify?id={id}&sig={sig}&pubkey={MYPUBKEY}