Make displayname colors based off of power level

Fixes #128

lib/helpers/extensions/displayname_color.dart

@@ -0,0 +1,13 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:material_ui/material_ui.dart";
+import "package:nexus/controllers/room_power_levels.dart";
+import "package:nexus/helpers/extensions/string_to_color.dart";
+
+extension DisplaynameColor on WidgetRef {
+  /// The color of [userId]'s name in [roomId], hashed from their power level,
+  /// so everyone with the same level shares one.
+  Color displaynameColor(String roomId, String userId) => watch(
+    RoomPowerLevelsController.provider(roomId)
+        .select((levels) => levels.levelOf(userId)),
+  ).toString().colorHash;
+}