fix for bug 985737, relies on PR 82 on legal-docs

This is a bit of a hack, rewriting URLs for all uses of {% fetch %}, but for the time being the only places we use {% fetch %} is for legal docs that need their URLs rewritten. Happy to field suggestions for cleaner ways of doing this.

Note that I have tested the URL remapping manually on the legal docs in github, but need those to be [deployed](https://github.com/mozilla/legal-docs/pull/82) to the CDN before it can actually be tested in fireplace properly.