add UserPageLink component

src/client/js/components/Common/UserPageLink.jsx

@@ -0,0 +1,22 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+
+export default class UserPageLink extends React.Component {
+
+  render() {
+    const { user } = this.props;
+
+    const name = user.name || '(no name)';
+    const username = user.username;
+    const href = `/user/${user.username}`;
+
+    return (
+      <a href={href}>{name} (@{username})</a>
+    );
+  }
+
+}
+
+UserPageLink.propTypes = {
+  user: PropTypes.object.isRequired,
+};