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/models/capabilities.dart

@@ -0,0 +1,25 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+
+part "capabilities.freezed.dart";
+part "capabilities.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const Capabilities({
+  @JsonKey(name: "org.matrix.msc4174.webpush") final WebPush? webpush,
+}) with _$Capabilities {
+  Map<String, Object?> toJson() => _$CapabilitiesToJson(this);
+
+  factory Capabilities.fromJson(Map<String, Object?> json) =>
+      _$CapabilitiesFromJson(json);
+}
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const WebPush({required final bool enabled, final String? vapid})
+    with _$WebPush {
+  Map<String, Object?> toJson() => _$WebPushToJson(this);
+
+  factory WebPush.fromJson(Map<String, Object?> json) =>
+      _$WebPushFromJson(json);
+}