GitLab

Created by: Razer6

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...