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/Markdown.js

@@ -99,7 +99,7 @@ export default class Markdown {
             // puts softbreaks in for multiple lines in a blockquote,
             // so if these are just newline characters then the
             // block quote ends up all on one line
-            // (https://github.com/vector-im/riot-web/issues/3154)
+            // (https://github.com/vector-im/element-web/issues/3154)
             softbreak: '<br />',
         });
 
@@ -166,7 +166,7 @@ export default class Markdown {
      * Render the markdown message to plain text. That is, essentially
      * just remove any backslashes escaping what would otherwise be
      * markdown syntax
-     * (to fix https://github.com/vector-im/riot-web/issues/2870).
+     * (to fix https://github.com/vector-im/element-web/issues/2870).
      *
      * N.B. this does **NOT** render arbitrary MD to plain text - only MD
      * which has no formatting.  Otherwise it emits HTML(!).