Replace Riot with Element in docs and comments

This only covers the simple cases of references to issues and repos. More
complex areas, such as deployment scripts, will be handled separately.

Part of https://github.com/vector-im/element-web/issues/14864

src/WidgetMessaging.js

@@ -76,7 +76,7 @@ export default class WidgetMessaging {
                     console.error(err._error);
                 }
                 // Potential XSS attack if 'msg' is not appropriately sanitized,
-                // as it is untrusted input by our parent window (which we assume is Riot).
+                // as it is untrusted input by our parent window (which we assume is Element).
                 // We can't aggressively sanitize [A-z0-9] since it might be a translation.
                 throw new Error(msg);
             }