Add prefix for certain quick actions in rich text editor

What does this MR do and why?

Add prefix for certain quick actions in RTE

In rich text editor, when picking a quick action from the autocomplete dropdown, add a prefix "@" for user mention quick actions, "~" for label related quick actions and "%" for milestone quick actions.

Changelog: added

Screenshots or screen recordings

Before	After
Symbols @, ~ and % aren't added automatically.	They are added automatically after selecting the quick action.
Screen Recording 2023-10-18 at 11.34.54.mov	Screen Recording 2023-10-18 at 11.32.14.mov

Symbol map:

Quick Action	Symbol
/label, /unlabel, /relabel	~
/assign, /unassign, /reassign, /cc, /assign_reviewer, /unassign_reviewer, /reassign_reviewer	@
/milestone	%

How to set up and validate locally

1. Open an MR. Use rich text editor in the comment box.
2. Try typing any of the commands above.
3. After selecting a command, the appropriate symbol should automatically be inserted and trigger a new suggestions dropdown.

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 #406722 (closed)