Reduce crawler load

> Look to set appropriate entry in robots.txt to stop crawlers hammering the site.
> Since we allow searching back across time, we can let bots look at lots of content that is otherwise not used (and isn't in caches). This can be quite expensive.

- Add `rel='nofollow'` on the links that do the filtering
- Add `X-Robots-Tag` header on historical pages

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/68707106)