Run create-react-app

We really wanted to make the students do this, but you can't run CRA in
a non-empty directory because Mark Zuckerberg is a big dummy. See
facebook/create-react-app#334

Author: Dan Roberts

.gitignore

@@ -0,0 +1,21 @@
+# See https://help.github.com/ignore-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+
+# testing
+/coverage
+
+# production
+/build
+
+# misc
+.DS_Store
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*