Allow admins to invite new users

* This refactors the email sending so that all the logic related to users and emails are now contained in one location.
* The Reset password screen has been updated from its ugly plain self to use the same layout as the login.
* An admin can now create a new account without specifying a password. A link is generated that can be given to the intended user to configure the password. If emails are configured, it also sends an email
* An admin can now create accounts that still require the user to verify their if the setting is enabled from the server settings. A link is generated that can be given to the intended user to configure the password. If emails are configured, it also sends an email.
* The above features can be used in conjunction: An email will have to verify their email through a link. Once verified, the user is redirected to setting the password.
* When an email has been verified OR a password has been set, users are now redirected to the login page with the email filled in and a success status message shown instead of a dedicated thank you page.

BTCPayServer/Models/AccountViewModels/SetPasswordViewModel.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace BTCPayServer.Models.AccountViewModels
+{
+    public class SetPasswordViewModel
+    {
+        [Required]
+        [EmailAddress]
+        public string Email { get; set; }
+        [Required]
+        [StringLength(100, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
+        [DataType(DataType.Password)]
+        public string Password { get; set; }
+
+        [DataType(DataType.Password)]
+        [Display(Name = "Confirm password")]
+        [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
+        public string ConfirmPassword { get; set; }
+
+        public string Code { get; set; }
+        public bool EmailSetInternally { get; set; }
+    }
+}