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 - more than happy to take this bit out if it makes implementing #80 harder

Closes #30
Closes #6 

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As you were, the confusion was mine. The values in this table are assigned by the rake task so it could just as easily be `nil`, `'All'` or `'Horse'`. We're currently ignoring anything without a category (although we're not excluding them when we count all the applications) so the naming's pretty arbitrary, we should pick whatever we think makes the most sense (and maybe document it?)