Add env var to enable/disable thumbnail generation

.env.dev.example

@@ -50,5 +50,8 @@ EMAIL_HOST_PASSWORD=emailpassword123
 EMAIL_USE_TLS=true
 EMAIL_USE_SSL=false
 
+# Thumbnails Generation
+ENABLE_THUMBNAIL_GENERATION=false
+
 # Set this to true when initializing certbot for domain, false when not
 CERTBOT_INIT=false