Calculate statistics and update the authorities list page

* Adds a new table for AuthorityStatsSummary
* Adds a rake task to calculate stats per authority as a whole and on a per category basis (based on data already loaded into the database)
* Uses some ruby magic (and a horrendous if statement) to make `percentage_#{attribute}` methods for the stats counts
* Fixes a bug where a single controllers to override in the theme was hardcoded, now loads everything ending in _controller.rb in the theme's controllers directory
* Overrides the authority list controller
* Example wire-up of authority list page
* Adds `.stats.overview` and `.stats.category(category_name)` instance methods to the `Authority` model (relies on the `rake hampshire:calculate_authority_stats` having been run)

Closes #30
Closes #6 

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