zulip-mobile: Give useful error message on SSL errors, e.g. self-signed certificate

If a user tries to connect to a server that uses a self-signing certificate be able to detect that and give a reasonable error (instead of generic one)

React Native does not allow us to differentiate or to override this. We do not want to patch it.

In RealmScreen.js if const serverSettings = await getServerSettings(realm); fails, call a native function that tries to connect to the same URL but ignoring SSL. If successful, then the server is using a self-signed certificate. Show a specific error detailing this.

The task can be worked by two people, one doing the Android work and another doing the iOS code.