Add invoice event severity

closes #1681
https://i.imgur.com/eyMO9M3.png

BTCPayServer.Data/Data/InvoiceEventData.cs

@@ -20,6 +20,7 @@ namespace BTCPayServer.Data
         }
 
         public string Message { get; set; }
+        public EventSeverity Severity { get; set; } = EventSeverity.Info;
 
         internal static void OnModelCreating(ModelBuilder builder)
         {
@@ -35,5 +36,25 @@ namespace BTCPayServer.Data
 #pragma warning restore CS0618
                 });
         }
+        
+        public enum EventSeverity
+        {
+            Info,
+            Error,
+            Success,
+            Warning
+        }
+        
+        public string GetCssClass()
+        {
+            return Severity switch
+            {
+                EventSeverity.Info => "info",
+                EventSeverity.Error => "danger",
+                EventSeverity.Success => "success",
+                EventSeverity.Warning => "warning",
+                _ => null
+            };
+        }
     }
 }