Fix error in mapping saved modal on non-site pages

It is possible to have saved_mapping_ids in your session, but to be viewing a
page which doesn't relate to a site, so @site isn't set. Before now, this would
mean seeing a 5xx error page.

I was able to reproduce this by saving mappings in one tab and quickly
refreshing the homepage in another. There are lots of other ways to trigger the
problem.
Merge pull request #226 from alphagov/fix_error_displaying_modal_on_non_site_page

Fix error in mapping saved modal on non-site pages