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 

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of course, I noticed this when I was halfway through changing it!