Allow markdown to be used in titles 🚀

What does this MR do and why?

Currently, any markdown is not processed in titles. There is a single_line_pipeline Banzai pipeline, but it doesn't include the markdown filter.

Create a new single_pipeline_markdown_pipeline that adds the appropriate markdown filters. There are cases where we don't want to use markdown, such as for labels. So it's better to have both pipelines.

This has been scoped to just issuables - issues, MRs, epics, etc.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Without the MR applied, navigate to an MR, and change the title to

   Markdown :smile: and 🍤 title _italic_ @root foo [test link](https://gitlab.com) #39+ `code`

   Once saved, the title should look like this:

   

2. With the MR applied, edit the MR title slightly, so that the HTML cache is busted. It should now display like this

   

   In particular, note that the italic, test link, and code are rendered markdown, and the @root user reference is no longer active.

Related to #476513 (closed)