Add workflow to auto merge PRs (#2485)

* Add workflow to auto merge PRs

* Remove the pull_request_review trigger

It will always fail for forks and it's unlikely that a PR review
will be the step before an automerge (usually the label comes after
an approval OR a check completes).
In cases where it would have triggered the workflow and a merge,
the user can always manually merge after their approval anyway.

.github/workflows/auto-merge.yaml

@@ -0,0 +1,35 @@
+name: automerge
+on:
+  pull_request_target:
+    types:
+      - labeled
+      - unlabeled
+      - synchronize
+      - opened
+      - edited
+      - ready_for_review
+      - reopened
+      - unlocked
+  check_suite:
+    types:
+      - completed
+  status: {}
+jobs:
+  automerge:
+    runs-on: ubuntu-latest
+    steps:
+      - name: automerge
+        uses: "JakeChampion/automerge-action@e2f6d39bda"
+        env:
+          MERGE_LABELS: ""
+          MERGE_METHOD: "merge"
+          MERGE_METHOD_LABELS: "automerge=merge,autosquash=squash"
+          MERGE_METHOD_LABEL_REQUIRED: "true"
+          # Will automatically update the head branch with
+          # base branch changes
+          # At the moment, it needs to be a separate label
+          UPDATE_LABELS: "autoupdate"
+          # This is OK because weaveworksbot only has write permissions
+          # if it were an admin, it might override the checks/reviews
+          # see pascalgn/automerge-action#65
+          GITHUB_TOKEN: "${{ secrets.WEAVEWORKSBOT_TOKEN }}"