Do not make banned channels' usernames clickable

Currently when a channel is banned, it's username is [posted](https://github.com/aiogram/bot/blob/586b368858f657bb9cc73a149d653455b3edec93/aiogram_bot/handlers/simple_admin.py#L42) with leading @, making it clickable, which helps promoting such channels.
I suggest removing the @ symbol so chat user won't be able to open that channel instantly.

Later another rule can be applied which replaces some symbols in usernames with asterisks (*)