1197 login logout

This includes both #36 and #37 so those should be merged in before this pull request.

* I set up a gulp task to concatenate all the JS files into a single file, so we would not have to keep adding script tags to index.html.
* App uses the new `/session` endpoint to retrieve the currently logged in user.
* Nav menu hides / shows relevant fields based on whether the user is logged in.
* Logging out unsets the current user in memory after the user has been logged out on the server.