Detect embeddable URL in the RTE and offer to convert

What does this MR do and why?

Detects embeddable URLs in the RTE, and offers to convert them to embeds.

Screenshots or screen recordings

What	👀
When a link looks embeddable
.. and then you click Embed link
When a link doesn't look embeddable

How to set up and validate locally

1. Check out master.
2. Enable the allow_iframes_in_markdown FF.
3. Go to Admin → Settings → General → Embedded content.
4. Check "Enable embedded content", "YouTube", and "Figma", then click "Save changes".
5. Restart your Caproni cluster/GDK.
6. Open the rich-text editor somewhere, like a work item/MR description, or a comment.
7. Type a YouTube link, like https://www.youtube.com/watch?v=Ddy20YF3qR0 or https://youtu.be/WDDS4TivbuE. (After you type the link in, pressing space is what actually turns it into a link. If you paste it in, it'll be turned into a link right away.)
8. Move the cursor back over the link with the keyboard, or by clicking it.
9. You should see the "Embed link" icon in the floating menu that appears:
   
10. Click it!
11. Save the field and try playing the video.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.