Create App Flow for in-app authentication

[DSSoftware]

In order to use ULS in apps, you need to port all GUIs to app to render API responses. So to make it easier, App Flow should be created.

Possible flow:

• Register an authentication request (Returns ID, request secret, timestamp)
• Redirect user to authentication page.
• After user gives consent
  • Return token, request ID and timestamp to app.
  • After checking ID, continue.
• After something else happens (declined or timeout)
  • Return error
  • Ask user to authorize again.