MM-52438 Clean up web app CI and only run when web app code is changed (#23232)

* MM-52438 Only run web app CI for PRs with web app changes

* MM-52438 Don't wait to run web app tests in CI

* MM-52438 Remove commented out caching

* MM-52438 Reuse steps to set up webapp environment

* MM-52438 Add test-ci script to root package.json

* Simplify path filter

.github/workflows/webapp-ci-pr.yml

@@ -1,6 +1,8 @@
 name: Web App CI PR
 on:
   pull_request:
+    paths:
+      - 'webapp/**'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}