Update room tags behaviour to match spec more

Fix Riot's behaviour with room tags after my cleanup in
https://github.com/matrix-org/matrix-doc/pull/1457 . Although, reading
it again, it's not clear how you're supposed to tell the difference
between a reverse-dns tag name and a legacy freeform text tag
(contains a '.'?) - I've left it detecting these as freeform text
for now.

src/stores/RoomListStore.js

@@ -194,6 +194,11 @@ class RoomListStore extends Store {
                     }
                 }
 
+                // ignore any m. tag names we don't know about
+                tagNames = tagNames.filter((t) => {
+                    return !t.startsWith('m.') || lists[t] !== undefined;
+                });
+
                 if (tagNames.length) {
                     for (let i = 0; i < tagNames.length; i++) {
                         const tagName = tagNames[i];