Open links externally by default on iOS

[gnprice]

When opening an external link, we have a choice whether to do so within the app or in an external browser.

Currently we do so following the upstream Flutter defaults documented at launchUrl (except on Android as described at #279). On iOS, that means opening an in-app browser.

But what we chose on zulip-mobile was for the default on iOS to be an external browser, for reasons discussed here in chat, and I think all the same reasoning still applies. So we should do that instead.

Separately:

• We should offer an option (again just as we do in zulip-mobile), so that this is only about the default. That's part of Store some client settings #97.