Make Danger more strict about Git trailers

This is based on
https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5197.
Previously Danger would allow trailers such as `Changelog: foo bar baz`,
even though those are not valid changelog trailers.

danger/changelog/Dangerfile

@@ -20,7 +20,7 @@ CATEGORIES = YAML
   .freeze
 
 def check_changelog_trailer(commit)
-  trailer = commit.message.match(/^Changelog:\s*(?<category>\w+)/)
+  trailer = commit.message.match(/^Changelog:\s*(?<category>.+)$/)
 
   return :missing if trailer.nil? || trailer[:category].nil?