Calculate statistics and update the authorities list page

WIP - have missed a bit

* 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
* View shows example usage for the new stats data/methods, expecting this to be replaced by #80 (more than happy to take this out if it will simplify pending feature merges)
* **pending** A better way to override controllers, which I only thought of while writing the PR, brb

Closes #30
Closes #6