Fix #1416 -- Add canonical geodata field (#1458)

* Fix #1416 -- Add canonical geodata field for events and subevents

* Add optional geocoding through OpenCageData

* Fix markup everywhere

* Add Leaflet map to geo coordinates

* Fix tests, add credits

* Fix spelling

src/tests/api/test_subevents.py

@@ -79,6 +79,8 @@ TEST_SUBEVENT_RES = {
     'id': 1,
     'variation_price_overrides': [],
     'location': None,
+    "geo_lat": None,
+    "geo_lon": None,
     'is_public': True,
     'item_price_overrides': [],
     'meta_data': {'type': 'Workshop'}