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/components/structures/TimelinePanel.js

@@ -1186,7 +1186,7 @@ const TimelinePanel = createReactClass({
             if (!timeline) {
                 // Somehow, it seems to be possible for live events to not have
                 // a timeline, even though that should not happen. :(
-                // https://github.com/vector-im/riot-web/issues/12120
+                // https://github.com/vector-im/element-web/issues/12120
                 console.warn(
                     `Event ${events[i].getId()} in room ${room.roomId} is live, ` +
                     `but it does not have a timeline`,