Preload autocomplete_sources/members

What does this MR do and why?

Implements preload of members data within the markdown editor when browser triggers idle event. Members endpoint is heavily cached to prevent too much pressure on the servers.

This MR is currently completely omitting the rich text editor. The way rich text editor is handling the autosuggestions is much more complex and will require considerable effort to implement.

Screenshots or screen recordings

Before	After

preloadMembers

How to set up and validate locally

1. enable FF

   echo 'Feature.enable(:preload_autocomplete_members_issues_mrs)' | rails c

2. Visit issue edit page eg http://gdk.test:3000/Commit451/lab-coat/-/issues/41
3. Check if the members data has been preloaded

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 #427437 (closed)