Rework the Cypress & Percy CI (#8691)

* Split Cypress out into its own workflow

* Improve PR Details job to use github-script and output labels

* Fix wrongly using github.ref in workflow_run actions which always refer to develop

* Update pr-details to be far more generic

* Tweak how we fill command-prefix

* Tweak cypress job to pass more params & fix if condition

* Bring in external changes

* Add docs

* Use new composite action, and an action to update a status check based on this workflow run

* Iterate approach

.github/workflows/tests.yml

@@ -35,3 +35,16 @@ jobs:
           path: |
             coverage
             !coverage/lcov-report
+
+  app-tests:
+    name: Element Web Integration Tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v3
+        with:
+          cache: 'yarn'
+
+      - name: Run tests
+        run: "./scripts/ci/app-tests.sh"