Make single newlines cause a <br> by default

This should match what people expect from a chat application much
better. The biggest reason not to do this, is because some people might
paste markdown documents. For those people there is now a /cmark
command, which disables most of our extensions to cmark, including the
newline behaviour. There is a long discussion on the Fediverse and on
Github linked below.

Mastodon https://fosstodon.org/@deepbluev7/109771668066978726
fixes #757