Add Percy Visual Tests

This is the easiest way to ensure that builds are actually doing what
they're supposed to do.

Author: jrjohnson

.github/workflows/ci.yaml

@@ -33,6 +33,10 @@ jobs:
       run: npm run lint:hbs
     - name: test
       run: npm run test:ember
+    - name: percy
+      run: npm run test:percy
+      env:
+        PERCY_TOKEN: ${{secrets.PERCY_TOKEN}}
 
   test-floating:
     name: Floating Dependencies