* UI: Additional improvements to the User Invitation flow
Closes#6224.
* Clear invitation token only after the user can sign in
Fixes "404 Error on Follow-Up Visits" of #6236.
* Minor spacing fix
* Update accordion button
* Server: Make sending email optional when adding user
Closes#6158.
* Generate custom invite token and store it in user blob
Closes btcpayserver/app/#46.
* QR code for user invite
Closes#6157.
* Text fix
* 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
