Generate unstable changelogs using towncrier (#1340)

Replace the current stack of hugo templates with a towncrier invocation. The main advantage of this is that it means that the "Changes since last release" section is consistent with the changelogs for the actual releases.

This also changes the release process so that the changelog is generated before tagging, which means that the thing tagged v1.5 is actually the v1.5 spec.

Fixes #908.

.github/workflows/main.yml

@@ -99,10 +99,32 @@ jobs:
           name: openapi-artifact
           path: openapi.tar.gz
 
+  generate-changelog:
+    name: "📢 Run towncrier for changelog"
+    # skip for builds of git tags
+    if: "!startsWith(github.ref, 'refs/tags/')"
+    runs-on: ubuntu-latest
+    steps:
+      - name: "📥 Source checkout"
+        uses: actions/checkout@v2
+      - name: "➕ Setup Python"
+        uses: actions/setup-python@v4
+      - name: "➕ Install towncrier"
+        run: "pip install 'towncrier'"
+      - name: "Generate changelog"
+        run: ./scripts/generate-changelog.sh vUNSTABLE
+      - name: "📤 Artifact upload"
+        uses: actions/upload-artifact@v2
+        with:
+          name: changelog-artifact
+          path: content/changelog/vUNSTABLE.md
+
   build-spec:
     name: "📖 Build the spec"
     runs-on: ubuntu-latest
-    needs: [calculate-baseurl, build-openapi]
+    needs: [calculate-baseurl, build-openapi, generate-changelog]
+    # run even if generate-changelog was skipped
+    if: ${{ always() }}
     steps:
       - name: "➕ Setup Node"
         uses: actions/setup-node@v2
@@ -119,9 +141,14 @@ jobs:
         run: |
           npm i
           npm run get-proposals
+      - name: "📥 Download generated changelog"
+        if: "needs.generate-changelog.result == 'success'"
+        uses: actions/download-artifact@v3
+        with:
+          name: changelog-artifact
+          path: content/changelog
       - name: "⚙️ hugo"
         run: hugo --baseURL "${{ needs.calculate-baseurl.outputs.baseURL }}" -d "spec"
-
       # We manually unpack the spec OpenAPI definition JSON to the website tree
       # to make it available to the world in a canonical place:
       # https://spec.matrix.org/latest/client-server-api/api.json