Facebook link preview contains html comments & quick-actions

Summary

(Summarize the bug encountered concisely)

On Facebook, and potentially Google, the rendered description in the link preview includes and <!-- --> html comments or /quick-actions that happen to be in the first parts of the preview.

Steps to reproduce

(How one can reproduce the issue - this is very important)

Create a new issue and put some HTML comments and and quick-actions on the first few lines, then share it through Facebook.

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behaviour, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug has been fixed in a more recent version)

Here's an issue that uses html comments and quick-actions in the first few lines: https://gitlab.com/emsenn/personal/issues/35 Here's Facebook's debugger preview of that link: https://developers.facebook.com/tools/debug/sharing/?q=https%3A%2F%2Fgitlab.com%2Femsenn%2Fpersonal%2Fissues%2F35

What is the current bug behavior?

(What actually happens)

HTML comments and quick-actions are rendered in Facebook link previews.

What is the expected correct behavior?

(What you should see instead)

I'd imagine the first non-commented or quick-action bit of text, sanitized of its markdown.

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

This bug happens on GitLab.com