Proper caching and mounting with Turbolinks 5

[phil-monroe]

Fixes #27

Changes:

Follow the pattern of react-rails and always mount components on turbolinks:load whether initial HTML exists or not.

Please ensure that:

• [ ] Changelog is updated if not a minor patch
• [ ] Ruby linting is ok: rubocop is all green
• [ ] Javascript linting is ok: cd javascript/webpacker_react-npm-module/ && yarn lint is all green
• [ ] Tests are all green

[phil-monroe]

I'm not really sure why the specs are failing... It seems webpack is compiling just fine so I'm not sure why webpacker is having issues finding the manifest.json.

[phil-monroe]

By forcing webpacker to previous commit SHAs I can see that webpacker#a858f8e causes things to break