Make `gfm` delegate to `Gitlab::Markdown.gfm` instead of `markdown`

Some usages of the `gfm` helper depend on the text not being rendered as
standard Markdown. Ugh.

app/helpers/gitlab_markdown_helper.rb

@@ -61,7 +61,7 @@ module GitlabMarkdownHelper
   # TODO (rspeicher): Remove all usages of this helper and just call `markdown`
   # with a custom pipeline depending on the content being rendered
   def gfm(text, options = {})
-    markdown(text, options)
+    Gitlab::Markdown.gfm(text, options)
   end
 
   def asciidoc(text)