Implement a popup authenticator

[pichfl]

This aims to be the foundation for replacing the deprecated and abandoned torii ember addon and the authenticator we deprecated that relied on it.

Scope of this PR / authenticator:

• Provide a base implementation
• Open a third party (oAuth1/2) login page in a managed popup
• Handle the authentication redirect and closing of the popup
• Make it easy to extend the authenticator to your needs (e.g. by adopting certain torii providers)

Not part of the scope of this PR / authenticator:

• Include and replicate all providers included by torii

Todo

• [x] Figure out if this is a sensible way forward
• [ ] Write tests
• [ ] Provide documentation
• [ ] Provide migration guide going from torii to this