client_apps: use script/build

Each client_app is now expected to provide a runnable script at
/client_apps/app_name/script/build that builds its JS assets.

Also, `npm install` will only be run if an npm `package.json` file
exists, so the app is free not to use npm if it doesn't need to.

Closes CNVS-15445

TEST PLAN
---- ----

  - run `bundle exec rake canvas:compile_assets` and make sure it works

Change-Id: I635c79234dbb2c890e73cd9889fac86bd68fdf53
Reviewed-on: https://gerrit.instructure.com/41195
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>
QA-Review: Ahmad Amireh <ahmad@instructure.com>