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/Utils/MiscUtils.cs

@@ -0,0 +1,24 @@
+using System;
+
+namespace PluralKit.Core
+{
+    public static class MiscUtils
+    {
+        public static bool TryMatchUri(string input, out Uri uri)
+        {
+            try
+            {
+                uri = new Uri(input);
+                if (!uri.IsAbsoluteUri || (uri.Scheme != "http" && uri.Scheme != "https")) 
+                    return false;
+            }
+            catch (UriFormatException)
+            {
+                uri = null;
+                return false;
+            }
+
+            return true;
+        }
+    }
+}