message: Fix block quotes all being on a single line
  Fixes
diff: @@ -92,7 +92,16 @@ export default class Markdown {
  }
  toHTML() {
  - const renderer = new commonmark.HtmlRenderer({safe: false});
  + const renderer = new commonmark.HtmlRenderer({
  + safe: false,
  +
  + // Set soft breaks to hard HTML breaks: commonmark
  + // puts softbreaks in for multiple lines in a blockquote,
  + // so if these are just newline characters then the
  + // block quote ends up all on one line
  + // (https://github.com/vector-im/riot-web/issues/3154)
  + softbreak: '<br />',
  + });
  const real_paragraph = renderer.paragraph;
  renderer.paragraph = function(node, entering) {
  