Add Danger CI check for changelog entries

This commit adds Danger via GitHub Actions. Dangerfiles are ruby files
that have a DSL for interacting with GitHub. They can do arbitrary
things. See: https://danger.systems/reference.html

The point of this commit is to automate the process of asking people to
update the changelog. This is a really really annoying thing that we
have to do too often. Editing a pull request will automatically re-run
the check.

Truly trivial commits can be marked as trivial easily by using the
hashtag trivial in the PR body. This is really just useful for actually
trivial things. Most commits actually do need to have associated
changelog entries.

.github/workflows/danger.yml

@@ -0,0 +1,21 @@
+name: Danger
+on:
+  workflow_dispatch:
+  pull_request:
+    types: [synchronize, edited, opened, reopened]
+jobs:
+  run:
+    name: Run danger checks
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 100
+        submodules: 'recursive'
+    - name: Install danger
+      run: |
+        sudo gem install danger
+    - name: Run danger
+      env:
+        DANGER_GITHUB_API_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: danger