Remove flutter chat (#26)

Had to squash merge manually as Forgejo was erroring

lib/models/content/membership.dart

@@ -0,0 +1,19 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/content/content.dart";
+import "package:nexus/models/membership_status.dart";
+part "membership.freezed.dart";
+part "membership.g.dart";
+
+@freezed
+abstract class MembershipContent extends Content with _$MembershipContent {
+  MembershipContent._();
+  factory MembershipContent({
+    @JsonKey(name: "displayname") required String? displayName,
+    @JsonKey(name: "membership") required MembershipStatus status,
+    Uri? avatarUrl,
+    String? reason,
+  }) = _MembershipContent;
+
+  factory MembershipContent.fromJson(Map<String, Object?> json) =>
+      _$MembershipContentFromJson(json);
+}