Add a warning for disallowed email domains (#29658)

Resolve #29660

Follow #29522 and #29609

Add a warning for disallowed email domains when admins manually add/edit
users.

Thanks @yp05327 for the
[comment](https://github.com/go-gitea/gitea/pull/29605#issuecomment-1980105119)

![image](https://github.com/go-gitea/gitea/assets/15528715/6737b221-a3a2-4180-9ef8-b846c10f96e0)

routers/web/admin/users.go

@@ -202,6 +202,11 @@ func NewUserPost(ctx *context.Context) {
 		}
 		return
 	}
+
+	if !user_model.IsEmailDomainAllowed(u.Email) {
+		ctx.Flash.Warning(ctx.Tr("form.email_domain_is_not_allowed", u.Email))
+	}
+
 	log.Trace("Account created by admin (%s): %s", ctx.Doer.Name, u.Name)
 
 	// Send email notification.
@@ -425,6 +430,9 @@ func EditUserPost(ctx *context.Context) {
 			}
 			return
 		}
+		if !user_model.IsEmailDomainAllowed(form.Email) {
+			ctx.Flash.Warning(ctx.Tr("form.email_domain_is_not_allowed", form.Email))
+		}
 	}
 
 	opts := &user_service.UpdateOptions{