build: use Actions to validate commit message

Actions interface has a better integration with GitHub, and with
Annotations and Problem Matcher we can display all failed checks in a
single place, so that users don't have to go through the logs to figure
out what's wrong. Since the job on Travis was allowed to fail and is not
as easy to read, remove it from our Matrix.

The Action will check every commit in the Pull Request, skipping commits
with "fixup" or "squash".

.github/workflows/commit-lint-problem-matcher.json

@@ -0,0 +1,13 @@
+{
+  "problemMatcher": [
+    {
+      "owner": "core-validate-commit",
+      "pattern": [
+        {
+          "regexp": "^not ok \\d+ (.*)$",
+          "message": 1
+        }
+      ]
+    }
+  ]
+}

.github/workflows/commit-lint.yml

@@ -0,0 +1,21 @@
+name: "Commit messages adheres to guidelines at https://goo.gl/p2fr5Q"
+
+on: [pull_request]
+
+jobs:
+  lint-commit-message:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          # Last 100 commits should be enough for a PR
+          fetch-depth: 100
+      - name: Use Node.js 12
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
+      - name: Validate commit messages
+        run: |
+          echo "::add-matcher::.github/workflows/commit-lint-problem-matcher.json"
+          git log --oneline ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }} | grep -v -e fixup -e squash | awk '{ print $1 }' |  xargs npx -q core-validate-commit --no-validate-metadata --tap