Draft: Add quick action to add thread when opening issuables

What does this MR do and why?

Related issues: #390661, #235345

Adds a /discussion <comment> quick action to add comment threads when creating issues/epics/merge requests. The primary use-case for this is in templates, so threads can be automatically created without user intervention.

Why is this only available during issuable creation?

1. If the issuable already exists, users can just comment/add a thread directly.
2. I don't want to figure out the best way to protect against cyclical comments (/discussion /discussion /discussion /discussion).
3. I don't want to decide on the best approach for using /discussion in comments on existing threads.

Why doesn't this match the designs in #390661 exactly?

1. &9447 got deprioritized as a whole, so I'm not really implementing that issue directly here. But this MR won't contradict anything described in #390661 & this MR would unblock that work if it ever does get prioritized again.
   • So then why am I even working on this? Sometimes brains are tricky and you just open MRs. Whoops, I guess?
2. The ~"<comment>" formatting/syntax isn't actually needed to post a single-line comment.
3. I didn't want to deal with expanding the command-identifying regex to allow for multiline comments.

Screenshots or screen recordings

Happy path: issues

Command search	Sample usage	Preview message	Posted issue

Unhappy path: comments

Empty command search	Lack of preview message	Error message on save & no comment from command-only note

How to set up and validate locally

1. Go through flow to create an issue, MR, or epic in any project/group
2. Include /discussion whatever comment in your description
3. Save!
4. See the beauty.

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.