[Javascript Testing] Add tests for StoryMedia

[FeminismIsAwesome]

Background:

We would like more confidence, especially with desires to update the UI and add new map-facing features, that the javascript is rendering as expected. This is a good place for those new to the project to start getting familiar with the javascript end of things.

Let's add tests for the StoryMedia.jsx component!

Some suggestions for tests:

Adding an enzyme snapshot test (https://medium.com/codeclan/testing-react-with-jest-and-enzyme-20505fec4675)
Asserting that different if-block renders render with the right data (media image vs audio image)
Testing interactions, like "when this clicks, this state changes"

Acceptance:

Running docker-compose exec web yarn test with the new tests does not break the javascript test suite.