Push Notifications (#66)

Fixes #35.

Things to do:
- [x] Register with local distributor
- [x] Register with homeserver
- [x] Recieve messages
- [x] Show notifications
- [ ] Notifications page (could come later)

Reviewed-on: Nexus/nexus#66

lib/controllers/push_key.dart

@@ -0,0 +1,30 @@
+import "dart:convert";
+
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/key.dart";
+
+class PushKeyController(final String instance) extends Notifier<String?> {
+  @override
+  String? build() => json.decode(
+    ref.watch(KeyController.provider(KeyController.pushKeyKey)) ?? "{}",
+  )[instance];
+
+  Future<void> set(String? value) async {
+    final provider = KeyController.provider(KeyController.pushKeyKey);
+    final notifier = ref.watch(provider.notifier);
+    final prefs = IMap(json.decode(ref.watch(provider) ?? "{}"));
+
+    state = value;
+    notifier.set(
+      json.encode(
+        prefs.add(instance, value).where((_, value) => value != null).unlock,
+      ),
+    );
+  }
+
+  static final provider =
+      NotifierProvider.family<PushKeyController, String?, String>(
+        PushKeyController.new,
+      );
+}