Switched to webpack in order to embed our own jquery, jquery-ui, select2, moment js libraries that become absolete in Nextcloud 19

Upgraded select2 from version 3 to version 4 in the process
https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/upgrade-guide.html#jquery-deprecation

Author: puthre

Makefile

@@ -85,7 +85,7 @@ endif
 .PHONY: npm
 npm:
 ifeq (,$(wildcard $(CURDIR)/package.json))
-	cd js && $(npm) run build
+	cd js && $(npm) install && $(npm) run build
 else
 	npm run build
 endif