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
