feat(webhooks): verify that url is accessible before saving it

PluralKit.Bot/Commands/Api.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 
@@ -126,6 +127,15 @@ namespace PluralKit.Bot
             if (_webhookRegex.IsMatch(newUrl))
                 throw new PKError("PluralKit does not currently support setting a Discord webhook URL as your system's webhook URL.");
 
+            try
+            {
+                await _dispatch.DoPostRequest(ctx.System.Id, newUrl, null, true);
+            }
+            catch (Exception e)
+            {
+                throw new PKError($"Could not verify that the new URL is working: {e.Message}");
+            }
+
             var newToken = StringUtils.GenerateToken();
 
             await _repo.UpdateSystem(ctx.System.Id, new()