Add SKIP_INTRODUCED_BY_URL_AUTOLOADING flag to speed up seeding in development

bin/seed takes hours to complete due to auto-loading (See #12)

This means, that you cannot just clone the project and follow the steps in How to develop locally without introducing a hack in the Loader class. This is a big hurdle for anybody unfamiliar with the codebase who wants to contribute.

This MR allows you to run bin/seed with the auto-loader disabled like this:

SKIP_INTRODUCED_BY_URL_AUTOLOADING=1 bin/seed

On my machine the scripts returns in ~90 seconds.

The trade-off by disabling the auto-loader is that not all metrics will have an "Introduced by URL" in the UI.