Allow user to create CensusAreas for new cities

[jeancochrane]

Update the CensusArea create view to allow the user to add CensusAreas for new cities:

1. Create a query param that filters the geographies and centers the map depending on the selected city
2. Create an autocomplete search box for dropdowns that reloads the page with new geographies when the user selects a new city
3. (Optional, depending on usability) Add a button like "Create a new geography in this city" to allow the user to circumvent the city selection process when creating lots of geographies

This will augment the work in #220 and allow new CensusAreas to be useable.