(release) Add Django changelog page

Expose the repository CHANGELOG.md through a staff-only Django view and
link it from the main navigation.

Render a small safe subset of Markdown without adding a runtime dependency,
copy the changelog into the Docker image, and cover the page with view tests.

Dockerfile

@@ -10,7 +10,7 @@ RUN apt-get update \
 
 WORKDIR /app
 
-COPY pyproject.toml README.md ./
+COPY pyproject.toml README.md CHANGELOG.md ./
 COPY src ./src
 COPY manage.py ./
 COPY scripts/docker-entrypoint ./scripts/docker-entrypoint