message: Transform h1 and h2 tags to h3 tags
  Addresses vector-im/riot-web#1772
diff: @@ -165,6 +165,12 @@ var sanitizeHtmlParams = {
  attribs.rel = 'noopener'; // https://mathiasbynens.github.io/rel-noopener/
  return { tagName: tagName, attribs : attribs };
  },
  + 'h1': function(tagName, attribs) {
  + return { tagName: 'h3', attribs: attribs };
  + },
  + 'h2': function(tagName, attribs) {
  + return { tagName: 'h3', attribs: attribs };
  + },
  '*': function(tagName, attribs) {
  // Delete any style previously assigned, style is an allowedTag for font and span
  // because attributes are stripped after transforming
  