Adjust PAIA auth login to OAuth 2.0 Token Endoint

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To make PAIA auth OAuth 2.0 compliant (#15), PAIA auth login should be extended to an OAuth 2.0 Token Endoint:

• require support of application/x-www-form-urlencoded (#50)
• support grant type authorization_code and request parameters
  • code (authorization code returned from authorization endpoint response)
  • redirect_uri (optional URL to redirect after authorization).
  • client_id and client_secret to authentificate the application (alternative to username and password for grant_type=password)
• support refresh_token (optional)
• make grant_type=password optional

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PAIA 1.4 wil optionally support client credentials grant. Support of grant type authorization_code would be nice but has not been requested so far.