Add limit in how many configurations each user may have. (#47)

* Add limit in how many configurations each user may have.

If the option max-number-client-config is more than 0 this number is the
maximum number of clients a user can create.

The setting only limits creation. If a user had created more
configurations before this setting is enforced or lowered the user may
user the service as before, just cant create any more configurations.

* Fix spelling and fmt as suggested by @luna-duclos

* Alert user when limit is reached.

When the user tries to create more configurations than are allow an
alert will pop up.

* Change http status as discussed with @freddd

http 400 seems a better fit than 429 as a more generic error.

ui/src/Clients.svelte

@@ -17,12 +17,22 @@
   async function handleNewClick(event) {
     const res = await fetch(clientsUrl, {
       method: "POST",
-    });
-    let newClient = await res.json();
-    console.log("New client added", newClient);
+    })
+      .then(response => {
+        return response.json()
+    })
+    .then(data => {
+      if (typeof data.Error != "undefined") {
+          console.log(data.Error);
+          alert(data.Error);
+    } else {
+        console.log("New client added", data);
+    }
+  });
     await getClients();
   }
 
+
 	onMount(getClients);
 </script>