login: Give useful hints, format help, and error feedback in UI

[gnprice]

This will be a followup to #11, where I think for the first iteration we'll leave this aspect out, or at least incomplete.

When the user is trying to log into an account, we should:

• accept a URL with the leading https:// left implicit;
• give useful hints about what the "Zulip server URL" field is asking for;
• give useful feedback in the UI when the URL is ill-formed;
• give useful feedback when the URL isn't of a Zulip server, or the server can't be reached;
• give useful feedback when the email they try to enter isn't in the shape of an email;
• give useful feedback when the server doesn't accept the login;
• perhaps more in the same vein.

In particular we should bring this to at least the same level of helpfulness as we currently have in the RN app.