Fix TemplateProcessor stylesheet detection
Fix TemplateProcessor stylesheet detection

Ensures that the stylesheets in the tarball build get relative links to
their assets.

robinwhittleton/govuk_template,froddd/govuk_template,alphagov/govuk_template,froddd/govuk_template,froddd/govuk_template,jonheslop/govuk_template,roc/govuk_template,quis/govuk_template,daveraine/govuk_template,jonheslop/govuk_template,alphagov/govuk_template,froddd/govuk_template,quis/govuk_template,alphagov/govuk_template,bitzesty/govuk_template,alphagov/govuk_template,quis/govuk_template,froddd/govuk_template,roc/govuk_template,jonheslop/govuk_template,robinwhittleton/govuk_template,robinwhittleton/govuk_template,bitzesty/govuk_template,daveraine/govuk_template,daveraine/govuk_template,quis/govuk_template,alphagov/govuk_template,quis/govuk_template,roc/govuk_template,daveraine/govuk_template,bitzesty/govuk_template,jonheslop/govuk_template,jonheslop/govuk_template,robinwhittleton/govuk_template,robinwhittleton/govuk_template