Load only paths, not full mappings, for sitemaps
Load only paths, not full mappings, for sitemaps

This avoids loading mapping objects into memory when we only need
the paths. Numbers from dev with 4GB RAM, using a single unicorn,
after 10 requests:

master: 5-5.5s request time, 8.5% RAM
here: 3.5s request time, 4% RAM

alphagov/bouncer,alphagov/bouncer,alphagov/bouncer