Initial implementation of FTUE user lists design

This covers the "recents" section and rough design exclusively. It is known that the Field does nothing and that there's a bunch of missing functionality - this is to be iterated upon in future PRs. Labs flag is to aide development and should be removed in a very near future PR.

Also, this is focusing on DMs and not user lists in general because I misinterpreted the scope. I'll fix this in a future PR and instead make this the best DM invite dialog it can be.

Closes https://github.com/vector-im/riot-web/issues/11197

src/settings/Settings.js

@@ -128,6 +128,12 @@ export const SETTINGS = {
         supportedLevels: LEVELS_FEATURE,
         default: false,
     },
+    "feature_ftue_dms": {
+        isFeature: true,
+        displayName: _td("New DM invite dialog (under development)"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "mjolnirRooms": {
         supportedLevels: ['account'],
         default: [],