`oauth_login`: triggered after successful authentication via OAuth, providing token and identity information
`oauth_refresh_token`: triggered after the OAuth access token was refreshed
... as suggested in issue #6933
* add config option `oauth_verify_peer`
* add config option `oauth_identity_fields`
* do not mask access token in session data
* fix refresh token handling
* use a redirect URL without query parameters
- Add "Login with XXX" button to login screen if oauth is configured
- Perform OAuth login procedure and get an access token
- Implement XOAUTH2 authentication type for IAMP and SMTP
Requires a patched and not yet released version of Net_SMTP.