Support <details> in GitHub notifications

I just received a GitHub notification from dependabot with a lot of HTML shown as plaintext in Element, even though the tags themselves are usually supported.

The big one is `` which may the culprit, since it encases most of the other stuff.

Other tags are p, em, a, blockquote, h2, h3, h4, pre code, ul, li, ...

Start by tracing how GitHub notification HTML is parsed and rendered, then inspect the existing handling for tags such as p, em, blockquote, and pre code. Confirm the behavior with a Dependabot notification containing details content; done means the supported content renders as HTML rather than appearing as plaintext.

Written by the indexing model from the issue text.