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/netlify.yaml

@@ -3,7 +3,7 @@
 name: Upload Preview Build to Netlify
 on:
   workflow_run:
-    workflows: [ "Element Web - Build and Test" ]
+    workflows: [ "Element Web - Build" ]
     types:
       - completed
 jobs: