Pass iframe dimensions through if supplied

What does this MR do and why?

Per #282443 ("What Could Still Be Implemented"), let's apply the width/height attributes to embedded frames the same way we do images.

I've also extracted the spec fixtures into a separate file for cleanliness' sake.

References

Past work:

• Add Banzai filter for detecting potential `ifra... (!204952 - merged) was the preliminary work for potential iframe recognition
• Allow iframe embeds in markdown from allowed sr... (!200864 - merged) carried the initial implementation
• Push iframe feature flag to frontend based on p... (!215529 - merged) fixed two bugs that stopped it from being usable on GitLab.com; since this MR the feature is configured and usable on GitLab.com, but you need to enable the FF for your group (we have it enabled in groupknowledge)

Present work: a series of 4 stacked MRs for %18.10 to bring this closer to GA:

• Pass iframe dimensions through if supplied (!224624 - merged) — You are here
• Avoid iframes having to deal with asset proxy/l... (!224625 - merged)
• Treat `<iframe>` as `<img>` (!224626 - merged)
• Transform `<iframe>` URLs according to a registry (!223870 - merged)

Screenshots or screen recordings

How to set up and validate locally

1. Check out the branch.
2. Enable the allow_iframes_in_markdown feature on your GDK.
3. Enable embedding of www.youtube.com from /admin/application_settings/general#js-iframe-settings, and ensure Enable embedded content is turned on.
4. It may be wise to restart the GDK Rails instance here, for best chances of success.
5. Using the plain-text editor, insert content like seen in the screenshot above to a wiki page, issue description, MR comment, etc.:

   Here's my YouTube video at default size:
   
   ![](https://www.youtube.com/embed/DwIepRJgMF4)
   
   Here it is with a reasonable size applied:
   
   ![](https://www.youtube.com/embed/DwIepRJgMF4){width=800 height=600}

6. It should look as it does in the screenshot above. (The status quo is that both embeds would be the same size as the first.)

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