Onboarding: Invite new users (#5714)

* Server Users: More precise message when inviting users

This lets the admin who invited a new user know whether or not an email has been sent. If the SMTP server hasn't been set up, they need to share the invite link with the user.

* 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`

* Remove duplicate status message from views that use the wizard layout

* Auto-approve users created by an admin

* Notify admins via email if a new account requires approval

* Update wording

* Fix update user error

* Fix redirect to email confirmation in invite action

* Fix precondition checks after signup

* Improve admin notification

Send notification only if the user does not require email confirmation or when they confirmed their email address. Rationale: We want to inform admins only about qualified users and not annoy them with bot registrations.

* Allow approval alongside resending confirm email

* Use user email in log messages instead of ID

* Prevent unnecessary notification after email confirmation

* Use ApplicationUser type explicitly

* Fix after rebase

* Refactoring: Do not subclass UserRegisteredEvent

BTCPayServer/Models/AccountViewModels/SetPasswordViewModel.cs

@@ -19,5 +19,6 @@ namespace BTCPayServer.Models.AccountViewModels
 
         public string Code { get; set; }
         public bool EmailSetInternally { get; set; }
+        public bool HasPassword { get; set; }
     }
 }