Add jsx-loader

In order to use the JSX syntax with React and bundle the React
components into our JS as needed, we must parse the JSX syntax.
Configure webpack to process JSX and work with React

Several adjustments to the webpack configs allow easily using React and
JSX. React is loaded as a global, so it has to be marked as an external
if we still want to use a 'require' statement to assign it as a
 dependency. JSX has to be parsed by the jsx-loader. And we also want to
 .gitignore the generated bundle.