Big refactor: ts/ directory for all typescript, including react

Split out test-specific and general utility react components too.

And moved our test/legacy* files for the Style Guide into a styleguide/
subdirectory of test/.

I think we'll be able to live in this directory structure for a while.

ts/components/utility/BackboneWrapper.tsx

@@ -0,0 +1,69 @@
+import React from 'react';
+
+interface Props {
+  /** The View class, which will be instantiated then treated like a Backbone View */
+  readonly View: BackboneViewConstructor;
+  /** Options to be passed along to the view when constructed */
+  readonly options: object;
+}
+
+interface BackboneView {
+  remove: () => void;
+  render: () => void;
+  el: HTMLElement;
+}
+
+interface BackboneViewConstructor {
+    new (options: object): BackboneView;
+}
+
+/**
+ * Allows Backbone Views to be rendered inside of React (primarily for the Style Guide)
+ * while we slowly replace the internals of a given Backbone view with React.
+ */
+export class BackboneWrapper extends React.Component<Props, {}> {
+  protected el: Element | null = null;
+  protected view: BackboneView | null = null;
+
+  public componentWillUnmount() {
+    this.teardown();
+  }
+
+  public shouldComponentUpdate() {
+    // we're handling all updates manually
+    return false;
+  }
+
+  public render() {
+    return <div ref={this.setEl} />;
+  }
+
+  protected setEl = (element: HTMLDivElement | null) => {
+    this.el = element;
+    this.setup();
+  }
+
+  protected setup = () => {
+    const { el } = this;
+    const { View, options } = this.props;
+
+    if (!el) {
+      return;
+    }
+    this.view = new View(options);
+    this.view.render();
+
+    // It's important to let the view create its own root DOM element. This ensures that
+    //   its tagName property actually takes effect.
+    el.appendChild(this.view.el);
+  }
+
+  protected teardown() {
+    if (!this.view) {
+      return;
+    }
+
+    this.view.remove();
+    this.view = null;
+  }
+}