model: Add isFullMember helper method to User

Author: sm-sayedi

lib/api/model/model.dart

@@ -262,6 +262,13 @@ class User {
   factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
 
   Map<String, dynamic> toJson() => _$UserToJson(this);
+
+  // This is determined based on:
+  //   https://zulip.com/api/roles-and-permissions#determining-if-a-user-is-a-full-member
+  bool isFullMember(int realmWaitingPeriodThreshold) {
+    final dateJoined = DateTime.parse(this.dateJoined);
+    return DateTime.now().difference(dateJoined).inDays >= realmWaitingPeriodThreshold;
+  }
 }
 
 /// As in [User.profileData].