Filter issues by city

Right now, the map view centers at the user's location when geolocation is given, and defaults to Toronto when geolocation is blocked. I think we should add a filter on the sidebar to choose between cities that have issues in them. Then, selecting a city would change the map view to that city and load the survey data for that city. Some of the work needed to be done here would go along with the work for issue #14 (closed) (caching survey data by city).