Add display_units as a required variable (nightscout#4559)

app.json

@@ -95,7 +95,7 @@
     "DISPLAY_UNITS": {
       "description": "Preferred BG units for the site:'mg/dl' or 'mmol'. (Note that it is *not* 'mmol/L')",
       "value": "mg/dl",
-      "required": false
+      "required": true
     },
     "ENABLE": {
       "description": "Plugins to enable for your site. Must be a space-delimited, lower-case list. Include the word 'bridge' here if you are receiving data from the Dexcom Share service. Include 'mmconnect' if you are bridging from the MiniMed CareLink service.",