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/Modal.tsx

@@ -319,7 +319,7 @@ export class ModalManager {
 
     private reRender() {
         if (this.modals.length === 0 && !this.priorityModal && !this.staticModal) {
-            // If there is no modal to render, make all of Riot available
+            // If there is no modal to render, make all of Element available
             // to screen reader users again
             dis.dispatch({
                 action: 'aria_unhide_main_app',