GreenField: Notifications API (#2055)

* GreenField: Notifications API

This refactors notifications so that we dont have a bunch of duplicated direct access to db contexts in controllers and then introduces new endpoints to fetch/toggle seen/remove  notifications of the current user.

* add tests + docs

* fix test

* pr changes

* fix permission json

BTCPayServer.Client/Models/NotificationData.cs

@@ -0,0 +1,16 @@
+using System;
+using Newtonsoft.Json;
+
+namespace BTCPayServer.Client.Models
+{
+    public class NotificationData
+    {
+        public string Id { get; set; }
+        public string Body { get; set; }
+        public bool Seen { get; set; }
+        public Uri Link { get; set; }
+
+        [JsonConverter(typeof(NBitcoin.JsonConverters.DateTimeToUnixTimeConverter))]
+        public DateTimeOffset CreatedTime { get; set; }
+    }
+}