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.

[chrisbobbe]

(EDITED)

I've just filed new issues for further work in this direction:

• login: Interpret API errors for the user #105
• login: When requireEmailFormatUsernames, check client-side that username looks like an email address #106
• login: In realm-input, give "subdomain required" feedback, when servers should but don't #107
• login: Gracefully handle attempts to log into an account that's already logged in #108
• login: Link to doc to help users understand what a "server URL" is and how to find theirs #109
• login: Investigate highlighting CZO and Zulip Cloud realms in platform autofill #110
• login: Give tappable realm URL suggestions based on text typed so far #111

So, closing this. 🎉