add support for multiple elections on map display

[jamesturk]

• alter core.views.state_overview to send multiple elections
• alter templates/core/state_overview.html to send all elections through instead of passing just the first as a property (use https://docs.djangoproject.com/en/2.2/ref/templates/builtins/#json-script as we do elsewhere)
• alter precinct-map.js to provide toggle between the newly available data