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/conversation/Message.tsx

@@ -0,0 +1,36 @@
+import React from 'react';
+
+
+/**
+ * A placeholder Message component, giving the structure of a plain message with none of
+ * the dynamic functionality. We can build off of this going forward.
+ */
+export class Message extends React.Component<{}, {}> {
+  public render() {
+    return (
+      <li className="entry outgoing sent delivered">
+        <span className="avatar" />
+        <div className="bubble">
+          <div className="sender" dir="auto" />
+          <div className="attachments" />
+          <p className="content" dir="auto">
+            <span className="body">
+              Hi there. How are you doing? Feeling pretty good? Awesome.
+            </span>
+          </p>
+          <div className="meta">
+            <span
+              className="timestamp"
+              data-timestamp="1522800995425"
+              title="Tue, Apr 3, 2018 5:16 PM"
+            >
+              1 minute ago
+            </span>
+            <span className="status hide" />
+            <span className="timer" />
+          </div>
+        </div>
+      </li>
+    );
+  }
+}