Render Kramdown through Gitlab Markup

In this commit we change the way Kramdown format is rendered.

Instead of using `GitHub::Markup` we're going to use `GitLab::Markup`,
avoiding rendering some extensions Kramdown has.

changelogs/unreleased/fj-render-kramdown-through-gitlab-markup.yml

+---
+title: Render Kramdown format using Gitlab markup
+merge_request: 56750
+author:
+type: changed

lib/gitlab/markup_helper.rb

@@ -4,7 +4,7 @@ module Gitlab
   module MarkupHelper
     extend self
 
-    MARKDOWN_EXTENSIONS = %w[mdown mkd mkdn md markdown].freeze
+    MARKDOWN_EXTENSIONS = %w[mdown mkd mkdn md markdown rmd].freeze
     ASCIIDOC_EXTENSIONS = %w[adoc ad asciidoc].freeze
     OTHER_EXTENSIONS = %w[textile rdoc org creole wiki mediawiki rst].freeze
     EXTENSIONS = MARKDOWN_EXTENSIONS + ASCIIDOC_EXTENSIONS + OTHER_EXTENSIONS

spec/helpers/markup_helper_spec.rb

@@ -382,6 +382,27 @@ RSpec.describe MarkupHelper do
       end
     end
 
+    context 'when file is Kramdown' do
+      let(:extension) { 'rmd' }
+      let(:content) do
+        <<-EOF
+{::options parse_block_html="true" /}
+
+<div>
+FooBar
+</div>
+        EOF
+      end
+
+      it 'renders using #markdown_unsafe helper method' do
+        expect(helper).to receive(:markdown_unsafe).with(content, context)
+
+        result = helper.render_wiki_content(wiki)
+
+        expect(result).to be_empty
+      end
+    end
+
     context 'any other format' do
       let(:extension) { 'foo' }