Flow2: Refactor UI into Flow2AuthWidget only and improve Flow2Auth

Flow2: Refactor UI into Flow2AuthWidget only and improve Flow2Auth

- Flow2AuthCredsPage:
  - Remove .ui file and embed Flow2AuthWidget into layout

- Flow2AuthWidget:
  - Make use generic for Flow2AuthCredsPage and WebFlowCredentialsDialog
  - Fix _errorLabel to render HTML tags instead of dumping them as plain text

- Flow2Auth:
  - Explicitly start auth with startAuth(account) instead of using constructor
  - Take control of copying the auth link to clipboard
  - Request a new auth link on copying, to avoid expiry invalidation
  - Use signals statusChanged() and result() to be more verbose (status, errors)
  - Change timer invocation and add safety bool's to avoid weird behaviour when
    the user triggers multiple link-copy calls (fetchNewToken)

Signed-off-by: Michael Schuster <michael@schuster.ms>