Phase out opening_hours:covid19 field (#401)

…by making the field an optional field which is not displayed by default when selecting a preset. This approach allows mapeprs to easily see now potentially outdated pandemic opening hours and check/update/fix/convert them accordinly.

data/presets/emergency/lifeguard.json

@@ -4,7 +4,9 @@
         "ref",
         "operator",
         "building_area",
-        "opening_hours",
+        "opening_hours"
+    ],
+    "moreFields": [
         "opening_hours/covid19"
     ],
     "geometry": [