add invite support

Closes #82

lib/models/matrix_event.dart

@@ -0,0 +1,28 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/content/content.dart";
+import "package:nexus/models/epoch_date_time_converter.dart";
+
+part "matrix_event.freezed.dart";
+part "matrix_event.g.dart";
+
+/// A plain Matrix event, as sent by the server (e.g. stripped invite state).
+///
+/// Unlike [Event], this isn't stored in the gomuks database, so it has no row
+/// IDs or local content.
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable(createToJson: false)
+class const MatrixEvent({
+  required final String type,
+  final String? stateKey,
+  final String? sender,
+  final String? eventId,
+  final String? roomId,
+  @JsonKey(name: "origin_server_ts")
+  @EpochDateTimeConverter()
+  final DateTime? timestamp,
+  required final Content content,
+}) with _$MatrixEvent {
+  factory MatrixEvent.fromJson(Map<String, dynamic> json) =>
+      _$MatrixEventFromJson(json)
+          .copyWith(content: .fromEventJson(json["content"], json["type"]));
+}