Factor out Emoji parsing using html-pipeline-gitlab

This PR is one of the first PRs refactoring the markup handling. As discussed with @randx, I'll do this with small PRs, to keep changes detectable.

### What it does?

It removes parsing emoji references, such as `:+1:`, from GitLab and calls instead `HTML::Pipeline::GitLab::MarkdownPipeline.` This pipeline currently only contains the  EmojiFilter.

In future we need to separate the markdown handling, by introducing different pipelines for different context. Eg.  for issues/MRs, dashboard...

I just tested this locally with relative url `/gitlab` - works :)