Add a character limit to avatar URLs (256 for now)

PluralKit.Core/Utils/Limits.cs

@@ -10,7 +10,7 @@ namespace PluralKit.Core {
         public static readonly int MaxDescriptionLength = 1000;
         public static readonly int MaxMemberNameLength = 100; // Fair bit larger than MaxProxyNameLength for bookkeeping
         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;
         public static readonly int AvatarDimensionLimit = 1000;
     }