Add a loading indicator for suggestions in content editor

What does this MR do and why?

When displaying the suggestions dropdown, show a loading indicator while the data is being fetched.

See #379045 (comment 1330979589)

Screenshots or screen recordings

Screen_Recording_2023-03-29_at_17.52.50

How to set up and validate locally

1. Access content editor. Content editor (rich text editor) is available in wikis globally, and in issue/MR comments behind a feature flag :content_editor_on_issues. Click "Viewing markdown" at bottom right and select "Rich text" from the menu to switch to content editor.
2. Try to insert a reference to a user / issue or a quick action.
3. Notice that now there is a loading indicator for the duration the data is being fetched.

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.