Transform h1 and h2 tags to h3 tags

Addresses vector-im/riot-web#1772

Signed-off-by: Travis Ralston <travpc@gmail.com>

src/HtmlUtils.js

@@ -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