Make Rails code reloading configurable in development and test

For many frontend developers, code reloading is unnecessary and creates more problems than it solves.

This commit adds a CACHE_CLASSES environment variable that can be defined to disable code reloading. We do this as an environment variable because this variable is most conveniently defined in the development.rb and isn't needed in production.

Relates to gitlab-development-kit#794 (closed)