Fix preview of some gif URLs

[blastrock]

Related to #11563

This PR fixes the preview of tenor.com URLs.

Before:

After:

This was broken because oEmbed JSON from tenor.com looks like:

{
  "type": "video",
  "version": "1.0",
  "provider_name": "Tenor",
  "provider_url": "https://tenor.com/",
  "cache_age": 86400,
  "html": "<iframe src=\"https://tenor.com/embed/4945328\" width=\"600\" height=\"400\" frameborder=\"0\"></iframe>",
  "width": 600,
  "height": 400
}

And we don't know how to handle the iframe. This PR fixes it by aborting the oEmbed preview and falling back to the Open Graph one (which I think was what was done before).

Note that this does not completely fix #11563 because imgur previews are still broken. The oEmbed looks like

{
  "version": "1.0",
  "type": "rich",
  "provider_name": "Imgur",
  "provider_url": "https://imgur.com",
  "width": 540,
  "height": 500,
  "html": "<blockquote class=\"imgur-embed-pub\" lang=\"en\" data-id=\"a/Ojsfo1t\"><a href=\"https://imgur.com/a/Ojsfo1t\">self-quarantine New Years Celebration</a></blockquote><script async src=\"//s.imgur.com/min/embed.js\" charset=\"utf-8\"></script>"
}

There are multiple issues here:

1. The type is "rich", so we can't know if there is supposed to be an image or video in that preview.
2. The html does not contain the preview per-se, we need to interpret the HTML and JavaScript (that script sources another script!) to render it.
3. The Open Graph response does not contain the preview, only the imgur logo itself.

So this PR will not fix imgur previews.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[clokep]

@blastrock I took another look -- it is a bit easier if you don't rebase / force push your work. We'll squash-merge it all down to one commit at the end, so the intermediate commits don't really matter.

[clokep]

Looks pretty good overall! I left some comments, but I think they're most nits / wording issues.

It seems tests are failing too?

[blastrock]

Thanks for the review! I agree with your comments, I just applied them.

I also fixed that one failing test, I only ran test_url_preview on my machine, I didn't see that one. Should be good now, I also ran the homeserver and generated some previews, everything seems fine.

[clokep]

I also fixed that one failing test, I only ran test_url_preview on my machine, I didn't see that one. Should be good now, I also ran the homeserver and generated some previews, everything seems fine.

Yeah the tests are not all in one place, unfortunately. 😢

[clokep]

I checked the results with a handful of sites and this seems good. 👍

[clokep]

LGTM! Thanks for working through this.

[blastrock]

Thanks for your help and reviews :)