Add asset size workflow (#7864)

This uses a fork of the Ember Asset Size Action to report on changes to
the filesizes of the production UI build as a comment to the PR. The
fork includes fixes to make it work with a subdirectory (ui/ in our
case) and to make it update the PR comment if it exists vs posting
repeatedly to the thread.

.github/workflows/ember-assets.yml

@@ -0,0 +1,24 @@
+name: Ember Asset Sizes
+
+on:
+  pull_request:
+    paths:
+    - 'ui/**'
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2-beta
+        with:
+          fetch-depth: 0
+      - name: Use Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: '10.x'
+      - uses: backspace/ember-asset-size-action@edit-comment
+        with:
+          repo-token: "${{ secrets.GITHUB_TOKEN }}"
+          working-directory: "ui"