Allow <details> and <summary> in suggested HTML subset
Specs [MSC2184](https://github.com/matrix-org/matrix-doc/pull/2184)
Based on
@@ -53,7 +53,7 @@ tags to permit, denying the use and rendering of anything else, is:
`font`, `del`, `h1`, `h2`, `h3`, `h4`, `h5`, `h6`, `blockquote`, `p`,
`a`, `ul`, `ol`, `sup`, `sub`, `li`, `b`, `i`, `u`, `strong`, `em`,
`strike`, `code`, `hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`,
-`th`, `td`, `caption`, `pre`, `span`, `img`.
+`th`, `td`, `caption`, `pre`, `span`, `img`, `details`, `summary`.
Not all attributes on those tags should be permitted as they may be
avenues for other disruption attempts, such as adding `onclick` handlers
