Use the stats partial for both the authority and search results

I've had a play at making a unified sidebar partial and while it's possible, I'm not sure it's actually better to have 1 complicated partial rather than 2 relatively simple one (open to other ideas though).

For now, I've broken the authority sidebar code out into its own partial and made the 2 sidebar partials' structure more similar.

The stats partial has been moved to `shared/_stats.html.haml` and is now shared between the 2.

(There may be scope to squash some commits here once this branch is finalised)

Closes #145 

<!---
@huboard:{"order":167.0,"milestone_order":169,"custom_state":""}
-->

Annoyingly, I think this will have to check for `@authority.blank?` because in the search context it could be, and so this sentence would look weird.