Prevent transient Capybara timeouts during feature tests

The problem occurred because asset compilation takes a long time, so when the asset cache didn't exist and the first test ran, it would often (randomly) time out during the generation before the actual test even had a chance to run.

Now we check if the cache exists before the suite runs, and if not, we manually fire a request to the root URL in order to generate it. This should allow subsequent tests to use the cached assets.