Handle all the segments of a v3 event ID

They may contain slashes, so it is not suitable to just pull the first segment after the room ID. Instead, we just recompile the event ID from known source, assuming everything afterwards is an event ID.

Fixes element-hq/element-web#8315

This will need adapting to support element-hq/element-web#9149

src/components/structures/MatrixChat.js

@@ -1561,7 +1561,16 @@ export default React.createClass({
         } else if (screen.indexOf('room/') == 0) {
             const segments = screen.substring(5).split('/');
             const roomString = segments[0];
-            const eventId = segments[1]; // undefined if no event id given
+            let eventId = segments.splice(1).join("/"); // empty string if no event id given
+
+            // Previously we pulled the eventID from the segments in such a way
+            // where if there was no eventId then we'd get undefined. However, we
+            // now do a splice and join to handle v3 event IDs which results in
+            // an empty string. To maintain our potential contract with the rest
+            // of the app, we coerce the eventId to be undefined where applicable.
+            if (!eventId) eventId = undefined;
+
+            // TODO: Handle encoded room/event IDs: https://github.com/vector-im/riot-web/issues/9149
 
             // FIXME: sort_out caseConsistency
             const thirdPartyInvite = {