Fix spurious html tags like <shrug>
Fix spurious html tags like <shrug>

 * Only render HTML tags in markdown if they're del tags
 * Consider non-allowed HTML tags as plain text nodes, so
   a message of just '<shrug>' doesn't need to be sent as
   HTML
 * Consequently rewrite isPlaintext to just look at the parse
   tree rather than making and gutting a renderer to walk
   the tree (now we're using a library that actually produces
   a meaningfgul parse tree).
 * Tweak when we put \n on text output to avoid putting \n on
   the end of messages.

Fixes https://github.com/vector-im/riot-web/issues/3065

matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk