One authenticated unit test

The PR compliment to #2581

Standard install and testing procedures to test.

This includes a `npm-shrinkwrap.json` due to a bug in `selenium-standalone` breaking `phantomjs`.
We should update [Makefile#L8][Makefile] too then, since it doesn't reference it there for unit tests either :P
[Makefile]: https://github.com/gratipay/gratipay.com/blob/8f13d711fd3cb9b1dc71b403416efc3e29d8d463/Makefile#L8