Use iframes instead of popups for saml authentication

[kptriescoding]

Is your feature request related to a problem? Please describe.
Alerta app uses popups for saml authentication. This approach faces problem due to popups being blocked in browsers by default and some users might be reluctant to allow popup.

Describe the solution you'd like
Please consider using iframes instead to prevent popups

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.