SAML2 Authentication - #1787

Continuation of #1576.

TODO

• Align config/env property names
• Align config prop names
• Testing of the core service & routes
• Update of .env.complete with available options.
• Add login icon, using a sensible name that can be overidden via the theme system if wished.
• Update readme with attribution of libs used

Questionables

• Do we need to consider multiple IDP usage? I'm tempted to say no for initial implementation but I'm worried we could be bitten by this. Maybe have a prefix to IDP-specific options now, Just as default?
  • Going to not worry about this for now, as we may be suiting the wrong requirement. Focus on single IDP.
• Do we need the laravel library? Tempted to test usage of the OneLogin lib directly just so we don't have to rely on anther laravel-specific lib which adds another consideration on framework updates.
  • Removal now started

Considerations

• Implementation of this will put pressure on the requirement of being able to disable the default auth. Don't want another large auth change in the release so will maybe need to queue that for release afterwards.
• Since email auth will still be in place, users could reset their password via normal means until the above has been implemented.