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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Ah fair point, I think at the moment we won't count uncategorised applications outside of this special "All" count so using `nil` to mean "no specific category" rather than "applications without a category" is ok. It's more in line with how I'd expect it to be used, since it's a quasi-relational field.

If you'd rather go the other way though, it raises the question as to whether the "All" stats should be stored in this table at all or live somewhere different, in another table perhaps, or in the Authority model as a method?