Add an API page, linking to PopIt and Popolo JSON exports

This depends on POPIT_API_URL being set in general.yml and nightly
mirroring to PopIt and dumps to Popolo JSON.

This is a partial fix for #1430 - the page would still need to be linked
to from elsewhere on the site as well (e.g. the nav bar and footer)
ZA: update installation instructions to refer to the generated Popolo JSON
Remove old Popolo JSON file

This is now provided by the API page
Merge branch 'add-api-page'