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/editor/deserialize.ts

@@ -158,7 +158,7 @@ function checkDescendInto(node) {
 
 function checkIgnored(n) {
     if (n.nodeType === Node.TEXT_NODE) {
-        // riot adds \n text nodes in a lot of places,
+        // Element adds \n text nodes in a lot of places,
         // which should be ignored
         return n.nodeValue === "\n";
     } else if (n.nodeType === Node.ELEMENT_NODE) {