Show email subject lines.

This includes a bunch of new things. I've introduced "ensureEmail..." to
indicate that the UI would like some data to be populated, but if it is
already present we don't need to do anything.

I've also introduced a cache for emails that is keyed on the email ID. I
don't know if email IDs are unique. They look like they should be
globally unique within a given server, but I'm not sure and the standard
is unclear. It'll need some experimentation.

src/EmailSummary.tsx

@@ -0,0 +1,40 @@
+import React from "react";
+import Client from "./client/Client";
+import { IAccount, IEmail } from "./client/types";
+
+type EmailSummaryProps = {
+	account: IAccount | null;
+	client: Client | null;
+	email: IEmail | null;
+	emailId: string;
+};
+type EmailSummaryState = {};
+
+class EmailSummary extends React.Component<
+	EmailSummaryProps,
+	EmailSummaryState
+> {
+	componentDidMount() {
+		this.ensureData();
+	}
+	componentDidUpdate() {
+		this.ensureData();
+	}
+
+	ensureData() {
+		if (this.props.account == null) return;
+		if (this.props.client == null) return;
+		this.props.client.ensureEmailGet(this.props.account.id, this.props.emailId);
+	}
+
+	render() {
+		return (
+			<div className="p-2" key={this.props.emailId}>
+				{this.props.email != null
+					? this.props.email.subject
+					: this.props.emailId}
+			</div>
+		);
+	}
+}
+export default EmailSummary;