Support initial deployment to Java 11 runtime

[wkurniawan07]

As part of https://github.com/TEAMMATES/teammates/projects/8

After all usages of App Engine APIs are removed, there are just a few more things left to be done in order to support deployment to Java 11 runtime:

• Add Jetty as web server framework
• Migrate appengine-web.xml, cron.xml, datastore-indexes.xml, queue.xml to their .yaml counterparts

This, however, is just the first milestone and does not mean that the new deployment will be ready for production usage. The new runtime has not been sufficiently studied yet at the point of writing.