Add support for quick actions in content editor

What does this MR do and why?

Add support for quick actions like /assign, /close /assign_reviewer in content editor in issue descriptions.

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag :content_editor_on_issues. To enable a feature flag, in terminal, enter rails c to open the Rails console and in the console enter Feature.enable(:content_editor_on_issues).
2. Open any project issue and edit its description.
3. Switch to Rich text editor (content editor).
4. Type / to bring up quick actions.
5. Press Enter to select any item. The quick actions will apply when saving.

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 #339483 (closed)