Resolve "Issue dropdown persists when adding issue number to issue description"

What does this MR do?

It fixes a problem that when the at.js autocomplete window in the markdown editor was open and then a save was submitted the window would stay open.

Solution : Destroying GLForm + GLAutocomplete when the markdown field is before being destroyed

Does this MR meet the acceptance criteria?

• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34403 (closed)