Better dev reloading

In development, we currently use shotgun if we want code reloading and thin if we want speed.

There are some problems with shotgun though:
* it's slow - it boots the entire app for every request
* it has a bug where doesn't log in development

mr-sparkles on the other hand, only reloads when files change, so is much faster.

Also this means using unicorn in development, which should be good for dev-prod parity.

I intend to make the same change in Content API if this PR is successful.

You can try it out with:
```bash
bundle exec mr-sparkle --force-polling -- -p 3009
```

Hat tip to @gma for suggesting mr-sparkle. :shipit: