Work around draft-js-export-html#62 by post-processing <br>\n

Fixes https://github.com/vector-im/riot-web/issues/4446 by post-processing the output HTML from draft-js-export-html by replacing `<br>\n` with `<br>`. This works for content within or outside of `<pre>`. If we replace with `\n` instead, the newlines only apply in `<pre>` tags so we use `<br>`.

src/components/views/rooms/MessageComposerInput.js

@@ -509,7 +509,7 @@ export default class MessageComposerInput extends React.Component {
         if (this.state.isRichtextEnabled) {
             contentHTML = HtmlUtils.stripParagraphs(
                 RichText.contentStateToHTML(contentState),
-            );
+            ).replace(/\<br\>\n/g, '<br>');
         } else {
             const md = new Markdown(contentText);
             if (md.isPlainText()) {