Add email link (passwordless) sign-in to <firebase-auth>

[FluorescentHallucinogen]

• Add signInWithEmailLink method.
• Add sendSignInLinkToEmail method.
• Add isSignInWithEmailLink method.
• Add fetchSignInMethodsForEmail method.

See https://firebase.google.com/docs/auth/web/email-link-auth for more info.

[FluorescentHallucinogen]

I know that the user object returned by <firebase-auth> already include these methods. But these methods are not listed in https://webcomponents.org/element/firebase/polymerfire/elements/firebase-auth#methods. How do users find out about them?

Currently the Polymer team is working on TypeScript type declarations. This should help e.g. with methods autocompletion. These declarations are automatically generated from Polymer elements source. So I believe that these methods should be declared in <firebase-auth> web component explicitly.

[FluorescentHallucinogen]

@tjmonsi PTAL.

[FluorescentHallucinogen]

@mbleigh @merlinnot PTAL.