Onboarding: Invite new users on store level (#5719)

* Onboarding: Invite new users

- Separates the user self-registration and invite cases
- Adds invitation email for users created by the admin
- Adds invitation tokens to verify user was invited
- Adds handler action for invite links
- Refactors `UserEventHostedService`
- Fixes #5726.

* Add permissioned form tag helper

* Better way of changing a user's role

* Test fixes

BTCPayServer/Services/Notifications/Blobs/InviteAcceptedNotification.cs

@@ -0,0 +1,53 @@
+using BTCPayServer.Abstractions.Contracts;
+using BTCPayServer.Configuration;
+using BTCPayServer.Controllers;
+using BTCPayServer.Data;
+using Microsoft.AspNetCore.Routing;
+
+namespace BTCPayServer.Services.Notifications.Blobs;
+
+internal class InviteAcceptedNotification : BaseNotification
+{
+    private const string TYPE = "inviteaccepted";
+    public string UserId { get; set; }
+    public string UserEmail { get; set; }
+    public string StoreId { get; set; }
+    public string StoreName { get; set; }
+    public override string Identifier => TYPE;
+    public override string NotificationType => TYPE;
+
+    public InviteAcceptedNotification()
+    {
+    }
+
+    public InviteAcceptedNotification(ApplicationUser user, StoreData store)
+    {
+        UserId = user.Id;
+        UserEmail = user.Email;
+        StoreId = store.Id;
+        StoreName = store.StoreName;
+    }
+
+    internal class Handler(LinkGenerator linkGenerator, BTCPayServerOptions options)
+        : NotificationHandler<InviteAcceptedNotification>
+    {
+        public override string NotificationType => TYPE;
+        public override (string identifier, string name)[] Meta
+        {
+            get
+            {
+                return [(TYPE, "User accepted invitation")];
+            }
+        }
+
+        protected override void FillViewModel(InviteAcceptedNotification notification, NotificationViewModel vm)
+        {
+            vm.Identifier = notification.Identifier;
+            vm.Type = notification.NotificationType;
+            vm.Body = $"User {notification.UserEmail} accepted the invite to {notification.StoreName}.";
+            vm.ActionLink = linkGenerator.GetPathByAction(nameof(UIStoresController.StoreUsers),
+                "UIStores",
+                new { storeId = notification.StoreId }, options.RootPath);
+        }
+    }
+}