Keep track of user's map view preferences

Description

As per #358 (closed)

The zoom level & location resetting every time the page is reloaded is really annoying (which sometimes happens unintentionally when the tablet puts Firefox to sleep). It should just keep the same state accross reloads

Scope

• Save map's zoom level and navigated coordinates localStorage
• On load,
  • set map's zoom level to the one previously used
  • if location unknown:
    • render map with last known zoom level and viewed coordinates
  • if location known:
    • render map with last known zoom level
    • if first map load within a single app's lifecycle, follow Whitebox marker
    • if consecutive map load, adjust to previously viewed coordinates