Fixes#18444. Inside of UrlPreviewer, we need to combine two dicts (one
from oEmbed, and one from OpenGraph metadata in the HTML) and in Mastodon's case they were very
different.
Single Page Applications (SPAs) seem to sometimes provide better information in the OpenGraph tags
than the oEmbed stubs, because the oEmbed stubs are filled in with JavaScript that Synapse does
not execute.
This change improves previews on Mastodon and YouTube (for the same reason).
Tested to not regress previews of Twitter or GitHub.
