Highlighting of person mentions now better visible on low contrast screens and…

What does this MR do?

Improves on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14377

Highlighting of person mentions now better visible on low contrast screens and consistent in size with other inline highlighting, hover also WCAG AA valid

Prepares for implementation of https://gitlab.com/gitlab-org/gitlab-ce/issues/49036 ⭐

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This feature wasn't really adding value due to it not being noticeable enough, especially on low contrast screens

Screenshots (if relevant)

old:

new:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?