issue #20614: build and serve client through the database

One of three pull requests (xtuple, xtuple-extensions, private-extensions)

The big architectural change is that the build client code is being served out of the database instead of out of the filesystem. The "build" and "deploy" folder are no longer used (except temporarily in the middle of the build process), and are deleted by the build process.

The API change for development and production is that the same build_app script that builds the database, now builds the client as well. I've added --clientonly and --databaseonly as possible flags in case the user doesn't want to build both sides at once.

Previous client-building solutions (deploy.sh, buildExtensions.sh, build_client.js) no longer do anything and have been deleted (except for deploy.sh, which is used at an intermediate step by this tool.)

Otherwise, everything should just work seamlessly, exactly as before.