Add post install/update scripts to simplify package management

[MikeAlhayek]

This simplify the package process. Instead of having to define a command to install each project, we use node script to find all project/theme that contains package.json and install/update it.

With this PR, we no longer need to update the root package.json to add a command for every project we want to build.

The new command node .scripts/install.js will be executed every time npm install in executed. It'll apply the same command on every project in the solution that contains package.json file.

If you want to execute npm update on all projects, you can execute npm run update-projects

If you want to run npm install on a specific project you may execute npm run install-project {ProjectName}. For example to install app packages in the TheAdmin theme only, you would run npm run install-project TheAdmin. Similarly, to update a project you can run npm run update-project TheAdmin