gitlab_markdown_helper.rb

The method Banzai::Renderer.pre_process would always be called,
regardless of whether the Markdown to render was already cached or not.
In cache the document _was_ cached the output of the pre-processing
pipeline was ignored resulting in it doing nothing but wasting CPU
cycles.

This commit moves Banzai::Renderer.pre_process into
Banzai::Renderer.render_result so that it's _only_ used when needed.