Allow deleting messages when erasing a user

Similarly to https://github.com/etkecc/synapse-admin/issues/23
It would be wonderful to have a toggle "Redact all user's messages" using the [New API endpoint](https://element-hq.github.io/synapse/v1.116/admin_api/user_admin_api.html#redact-all-the-events-of-a-user)