Fix toolbar buttons in Markdown field

What does this MR do and why?

Fixes the Markdown field toolbar buttons and their respective shortcuts that stop working after switching between Write and Preview modes.

What caused this bug?

Migrating the Editor’s Write and Preview tabs to the GlTabs component.

Screenshots or screen recordings

How to set up and validate locally

• Switch between Write and Preview modes in the Markdown field of your preference.
• Make sure the toolbar buttons and shortcuts still work.

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.