add knock support

Fixes #123

lib/helpers/extensions/join_room.dart

@@ -40,4 +40,29 @@ extension JoinRoom on BuildContext {
 
     return roomId;
   }
+
+  /// Knocks on a room, success and errors in snackbars.
+  /// Returns the room's ID, or null if knocking failed.
+  Future<String?> knockRoom(
+    JoinRoomRequest request, {
+    required String name,
+  }) async {
+    final container = ProviderScope.containerOf(this);
+    final messenger = ScaffoldMessenger.of(this);
+
+    final roomId = await withProgressSnackbar(
+      "Knocking on $name...",
+      () =>
+          container.read(ClientController.provider.notifier).knockRoom(request),
+    );
+    if (roomId == null) return null;
+
+    messenger.showSnackBar(
+      .new(
+        content: Text("Knocked on $name. Wait for a moderator to let you in."),
+      ),
+    );
+
+    return roomId;
+  }
 }

lib/helpers/extensions/show_user_popover.dart

@@ -1,14 +1,8 @@
 import "package:material_ui/material_ui.dart";
 import "package:nexus/helpers/extensions/show_adaptive_popover.dart";
-import "package:nexus/models/content/membership.dart";
 import "package:nexus/widgets/user_bottom_sheet.dart";
 
 extension ShowUserPopover on BuildContext {
-  void showUserPopover(
-    MembershipContent member,
-    String userId, {
-    String? roomId,
-  }) => showAdaptivePopover(
-    (context) => UserBottomSheet(member, userId, roomId: roomId),
-  );
+  void showUserPopover(String userId, {String? roomId}) =>
+      showAdaptivePopover((context) => UserBottomSheet(userId, roomId: roomId));
 }