Add a back to search link to application pages

This adds a link to application pages, that takes you back to the
search you came from, if the search params are present in the url.

These params are added to the query string of application links in
both the list and map views. They're whitelisted in both controllers
so that no XSS can happen through adding different parameters and
namespaced as search_xxx so that if we ever add url parameters to
application pages, these won't clash.

I'm not totally sure about this, as it feels a bit hacky. I guess trying to
add state to HTTP always is! I did look at saving the search url in the
session instead, but that would have required clearing it in every request too
and doing that seemed a bit invasive for this small change.

I'm prepared to find out that there's a very rails way to do this that I've
completely overlooked :)

Closes #171