Tweak Danger for the new changelog setup

We now only inform the user about the new setup if a changelog is
required or optional. In addition, the link to the rollout issue is now
wrapped in an inline code block. Using a regular link results in a lot
of system notes and related merge requests being created, making it
difficult to navigate the issue linked to.

danger/changelog/Dangerfile

@@ -13,6 +13,28 @@ merge_request: %<mr_iid>s
 #{SEE_DOC}
 SUGGEST_COMMENT
 
+CATEGORIES = YAML
+  .load_file(File.expand_path('../../.gitlab/changelog_config.yml', __dir__))
+  .fetch('categories')
+  .keys
+  .freeze
+
+def check_changelog_trailer(commit)
+  trailer = commit.message.match(/^Changelog:\s*(?<category>\w+)/)
+
+  return :missing if trailer.nil? || trailer[:category].nil?
+
+  category = trailer[:category]
+
+  return :valid if CATEGORIES.include?(category)
+
+  self.fail(
+    "Commit #{commit.sha} uses an invalid changelog category: #{category}"
+  )
+
+  :invalid
+end
+
 def check_changelog_yaml(path)
   raw_file = File.read(path)
   yaml = YAML.safe_load(raw_file)
@@ -75,7 +97,18 @@ elsif changelog.optional?
   message changelog.optional_text
 end
 
-message <<~MSG
+if changelog.required? || changelog.optional?
+  checked = 0
+
+  git.commits.each do |commit|
+    case check_changelog_trailer(commit)
+    when :valid, :invalid
+      checked += 1
+    end
+  end
+
+  if checked == 0
+    message <<~MSG
       We are in the process of rolling out a new workflow for adding changelog entries. This new workflow uses Git commit subjects and Git trailers to generate changelogs. This new approach will soon replace the current YAML based approach.
 
       To ease the transition process, we recommend you start using both the old and new approach in parallel. This is not required at this time, but will make it easier to transition to the new approach in the future. To do so, pick the commit that should go in the changelog and add a `Changelog` trailer to it.  For example:
@@ -96,4 +129,6 @@ message <<~MSG
       - https://docs.gitlab.com/ee/api/repositories.html#generate-changelog-data
 
       If you'd like to see the new approach in action, take a look at the commits in [the Omnibus repository](https://gitlab.com/gitlab-org/omnibus-gitlab/-/commits/master).
-MSG
+    MSG
+  end
+end