Further improvements to party data handling

This makes the following changes:

* Switch to getting Electoral Commission party data from @symroe's scraped
  version of their database, instead of the CSV export from their site.
* Upload the logos for parties to PopIt, which are in that repository.
* Don't fetch the party data from PopIt every time the framework is loaded;
  instead load some dumped JSON and add a command to generate that
  JSON.
* Fixes a bug in the Javascript for displaying the 2015 election select boxes
  when the person has no 2015 constituency selected.