Added auto-assign workflow to pull requests

no issue

- Renovate will open PRs to bump dependencies but these are unassigned
- Renovate does have the ability to set assignees, but these are blanket
  assigns not based upon the package that is being upgraded
- we want to assign them to the owners of the package that is being
  upgraded
- I've created the `gh-auto-assign` GitHub CLI exntension which reads a
  `.github/AUTO_ASSIGN` file and assign PRs based upon the entries listed
- this commit adds the initial `AUTO_ASSIGN` file and GitHub Actions
  workflow to trigger when a PR is opened

.github/AUTO_ASSIGN

@@ -0,0 +1,3 @@
+renovate/tryghost-kg-.* kevinansfield
+renovate/tryghost-members-.* allouis
+renovate/.* daniellockyer

.github/workflows/auto-assign.yml

@@ -0,0 +1,15 @@
+name: Auto Assign
+on:
+  pull_request:
+    types: [opened, reopened]
+jobs:
+  automate:
+    runs-on: ubuntu-18.04
+    env:
+      FORCE_COLOR: 1
+    steps:
+      - uses: actions/checkout@v2
+      - run: gh extension install daniellockyer/gh-auto-assign
+      - run: gh auto-assign
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}