login: Support web-based auth methods

[gnprice]

I expect the first iteration of a login flow #11 will offer only email/password auth. We should add support for the rest.

In particular this means web-based auth, which covers Google auth and GitHub auth among others. We can split out other methods as separate issues, if we don't find they're easy to just do at the same time. Other forms of auth are tracked separately:

• Dev login method #405
• Support Apple auth #462

[chrisbobbe]

For Apple, there's a "Flutter Favorite" plugin sign_in_with_apple.

Probably it makes sense to start with Apple; as I recall, they'll reject apps in App Store review if they have third-party auth but no Apple auth.

[chrisbobbe]

One wrinkle to keep in mind (as of writing): although authentication_methods is marked in the doc as deprecated in 2.1.0, it still seems necessary for writing a correct client. Discussion: https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/audit.20for.20change.20entries.20vs.2E.20central.20changelog/near/1404115

[gnprice]

For Apple, there's a "Flutter Favorite" plugin sign_in_with_apple.

Good to know, thanks.

Probably it makes sense to start with Apple; as I recall, they'll reject apps in App Store review if they have third-party auth but no Apple auth.

That means we'll need Apple auth before a full launch, or even an open beta in the form of an alternate app in the App Store. But I think we'll need web-based auth before that milestone, too, because lots of people rely on that.

One thing that could cause us to prioritize web-based auth or any other auth method sooner than that milestone is if it's an obstacle for a significant number of users who might join an earlier stage of the beta. I think that's unlikely for Apple auth, because I suspect not many people are using it, but is more likely for web-based auth because of people using Google or GitHub auth.