Add support for autocomplete suggestions in content editor

What does this MR do and why?

Add support for autocomplete suggestions for things like issues, MRs, users, milestones etc. in content editor.

Screenshots or screen recordings

Screen_Recording_2022-06-22_at_12.11.30_AM

How to set up and validate locally

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 #352667 (closed)