Add super basic group model/command

PluralKit.Core/Utils/Limits.cs

@@ -9,6 +9,7 @@ namespace PluralKit.Core {
         public static readonly int MaxMembersWarnThreshold = MaxMemberCount - 50;
         public static readonly int MaxDescriptionLength = 1000;
         public static readonly int MaxMemberNameLength = 100; // Fair bit larger than MaxProxyNameLength for bookkeeping
+        public static readonly int MaxGroupNameLength = 100;
         public static readonly int MaxPronounsLength = 100;
         public static readonly int MaxUriLength = 256; // May need to be set higher, I know there are URLs longer than this in prod (they can rehost, I guess...)
         public static readonly long AvatarFileSizeLimit = 1024 * 1024;