Re #423 - Collection Days: Home page - leaflet map, markers

• re-factored leaflet so that adding markers to map so that it can accommodate:
  - more than one marker
  - with a popup or not
  - popup open or not
• re-factored leaflet so that centre of map is handled separately from makrer placement
• re-factored lat_long, latLong variables so that they are consistently map_latlng
• added initial collection day markers