API patch improvements

- add PatchObject.CheckIsValid
- use transaction when creating member, as to not create a member if the
patch is invalid
- return edited system in `PATCH /s` endpoint

PluralKit.Core/Models/Patch/SystemPatch.cs

@@ -1,4 +1,6 @@
 #nullable enable
+using System.Text.RegularExpressions;
+
 namespace PluralKit.Core
 {
     public class SystemPatch: PatchObject
@@ -33,5 +35,14 @@ namespace PluralKit.Core
             .With("front_history_privacy", FrontHistoryPrivacy)
             .With("pings_enabled", PingsEnabled)
             .With("latch_timeout", LatchTimeout);
+
+        public new void CheckIsValid()
+        {
+            if (AvatarUrl.Value != null && !MiscUtils.TryMatchUri(AvatarUrl.Value, out var avatarUri))
+                throw new InvalidPatchException("avatar_url");
+            if (Color.Value != null && (!Regex.IsMatch(Color.Value, "^[0-9a-fA-F]{6}$")))
+                throw new InvalidPatchException("color");
+        }
+
     }
 }