Add party filter to home view

New flow:
  - Blank on first launch
  - User selects address
  - If error:
    - Hide GO! button
    - Hide party picker
    - Show error view and set error text
  - If partisan contests:
    - Display party picker
    - Display GO! button after user selects a party
    - Display GO! button automatically if user has already
      selected a party
  - If no partisan contests:
    - Display GO! button
      - Shift GO! button up to the space the party picker normally
        occupies
Add party filter choice to ballot view
Localize string dates in election view headers