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/Controllers/GreenField/GreenfieldStoreUsersController.cs

@@ -27,14 +27,15 @@ namespace BTCPayServer.Controllers.Greenfield
             _storeRepository = storeRepository;
             _userManager = userManager;
         }
-        [Authorize(Policy = Policies.CanModifyStoreSettings, AuthenticationSchemes = AuthenticationSchemes.Greenfield)]
+
+        [Authorize(Policy = Policies.CanViewStoreSettings, AuthenticationSchemes = AuthenticationSchemes.Greenfield)]
         [HttpGet("~/api/v1/stores/{storeId}/users")]
         public IActionResult GetStoreUsers()
         {
-
             var store = HttpContext.GetStoreData();
             return store == null ? StoreNotFound() : Ok(FromModel(store));
         }
+
         [Authorize(Policy = Policies.CanModifyStoreSettings, AuthenticationSchemes = AuthenticationSchemes.Greenfield)]
         [HttpDelete("~/api/v1/stores/{storeId}/users/{idOrEmail}")]
         public async Task<IActionResult> RemoveStoreUser(string storeId, string idOrEmail)