feat: put the access codes behind a feature flag

.env

@@ -77,3 +77,14 @@ EP_OAUTH2_URL_ACCESS_TOKEN=https://myep.ddev.site/token
 EP_OAUTH2_URL_USERINFO=https://myep.ddev.site/api/userinfo
 EP_OAUTH2_SCOPES=email,profile,roles
 ###< app/myep-oauth2 ###
+
+###> app/features ###
+# Feature flags, read by config/packages/flagception.yaml. Off here and switched on per
+# deployment in .env.local, so a feature reaches production when its environment says so rather
+# than when it is merged.
+#
+# Door, lift and barrier codes under /access-codes and their admin table. Off until the codes
+# have actually been entered: an empty list of codes is worse than no menu entry, because it
+# reads as "there are none" to somebody standing at the door.
+FEATURE_ACCESS_CODES=false
+###< app/features ###