Add post-connect UI to login via SASL

If the server supports SASL and if we aren't logged in with any
account, add a UI to authenticate via SASL. This allows users to
login anonymously then login via SASL.

This will also ease the draft/account-registration implementation.

components/auth-form.js

@@ -0,0 +1,51 @@
+import { html, Component } from "../lib/index.js";
+
+export default class NetworkForm extends Component {
+	state = {
+		username: "",
+		password: "",
+	};
+
+	constructor(props) {
+		super(props);
+
+		this.handleChange = this.handleChange.bind(this);
+		this.handleSubmit = this.handleSubmit.bind(this);
+
+		if (props.username) {
+			this.state.username = props.username;
+		}
+	}
+
+	handleChange(event) {
+		let target = event.target;
+		let value = target.type == "checkbox" ? target.checked : target.value;
+		this.setState({ [target.name]: value });
+	}
+
+	handleSubmit(event) {
+		event.preventDefault();
+
+		this.props.onSubmit(this.state.username, this.state.password);
+	}
+
+	render() {
+		return html`
+			<form onChange=${this.handleChange} onSubmit=${this.handleSubmit}>
+				<label>
+					Username:<br/>
+					<input type="username" name="username" value=${this.state.username} required/>
+				</label>
+				<br/><br/>
+
+				<label>
+					Password:<br/>
+					<input type="password" name="password" value=${this.state.password} required autofocus/>
+				</label>
+				<br/><br/>
+
+				<button>Login</button>
+			</form>
+		`;
+	}
+}