Update Jest config and remove unused node modules

Jest needs to know where to look for dependencies. It can also be
configured to read files with the '.jsx' and other extensions. It was
breaking on an error when trying to mock the 'statics-stylesheets'
library, and I wasn't even using that library, so I removed it along
with another unused library.

There is still an issue with Jest finding the required dependencies from
within a file. Jest seems to be using relative paths, while I have
webpack configured to use absolute paths from the project root
directory.