MD-escape URLs/alises/user IDs prior to parsing markdown

So that MD characters in them do not result in formatting being applied.

Fixes element-hq/element-web#3428
Fixes element-hq/element-web#4674

src/Markdown.js

@@ -55,14 +55,33 @@ function is_multi_line(node) {
     return par.firstChild != par.lastChild;
 }
 
+import linkifyMatrix from './linkify-matrix';
+import * as linkify from 'linkifyjs';
+linkifyMatrix(linkify);
+
+// Thieved from draft-js-export-markdown
+function escapeMarkdown(s) {
+    return s.replace(/[*_`]/g, '\\$&');
+}
+
+// Replace URLs, room aliases and user IDs with md-escaped URLs
+function linkifyMarkdown(s) {
+    const links = linkify.find(s);
+    links.forEach((l) => {
+        // This may replace several instances of `l.value` at once, but that's OK
+        s = s.replace(l.value, escapeMarkdown(l.value));
+    });
+    return s;
+}
+
 /**
  * Class that wraps commonmark, adding the ability to see whether
  * a given message actually uses any markdown syntax or whether
  * it's plain text.
  */
 export default class Markdown {
     constructor(input) {
-        this.input = input;
+        this.input = linkifyMarkdown(input);
 
         const parser = new commonmark.Parser();
         this.parsed = parser.parse(this.input);