Convert reference text to a reference after spacebar

What does this MR do and why?

Convert reference text to a reference after spacebar

Typing a username then pressing spacebar or clicking somewhere else in the Rich Text Editor textarea, isn't converted into a user mention. Similar issue exists for other references, which is now fixed by this commit.

Changelog: fixed

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Case	Before	After
Snippet	Screen Recording 2023-11-16 at 12.16.39.mov	Screen Recording 2023-11-16 at 12.11.46.mov
Milestone	Screen Recording 2023-11-16 at 12.16.58.mov	Screen Recording 2023-11-16 at 12.13.37.mov
User	Screen Recording 2023-11-16 at 12.16.07.mov	Screen Recording 2023-11-16 at 12.14.04.mov
Label	Screen Recording 2023-11-16 at 12.17.20.mov	Screen Recording 2023-11-16 at 12.14.23.mov
Vulnerability	Screen Recording 2023-11-16 at 12.18.51.mov	Screen Recording 2023-11-16 at 12.15.02.mov

How to set up and validate locally

1. In the comment box at the bottom of an issue, click "Switch to rich text editing".
2. As described in the videos, try to type in any valid user mention, label, milestone, snippet or vulnerability. Do not select anything from the dropdown.
3. After pressing space, the text should be automatically be converted to a reference link, if the reference is valid.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #419573 (closed)