Draw new icons for priority outdoor POIs

[nvkelso]

Replace generic sprites with custom artwork. Followup to #1.

• [x] trailhead (example)
• [x] ranger_station (example and 2)
• [x] caravan_site - RV campground (example)
• [x] picnic_table (example)
• [x] picnic_site - can have same icon as picnic_table (example)
• [x] monument – we can reuse icon from landmark, no special recolouring though (example)
• [x] bbq (example)
• [x] dog_park (example)
• [x] firepit (example)
• [x] swimming_area (example)
• [x] waterfall (example)
• [x] fishing_area (example) NOTE: was incorrectly specified here as fishing_spot before.
• [x] slipway - boat ramp (example)
• [x] shower (example)

[sensescape]

All of the above have icons, but the 3 unchecked example links don't show them: picnic_table, fishing_area and slipway

[sensescape]

Woot!