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/pretix/base/settings.py

@@ -704,6 +704,18 @@ Your {event} team"""))
         'default': '',
         'type': LazyI18nString
     },
+    'opencagedata_apikey': {
+        'default': None,
+        'type': str
+    },
+    'leaflet_tiles': {
+        'default': None,
+        'type': str
+    },
+    'leaflet_tiles_attribution': {
+        'default': None,
+        'type': str
+    },
     'frontpage_subevent_ordering': {
         'default': 'date_ascending',
         'type': str