Allow find_members to filter for multiple roles

Second attempt to fix !498 (closed) to make the Postorius index page hide non-members. This commit will allow you to specify multiple roles when searching for members, so Postorius can query for members, moderators and owners directly without the need to perform its own filtering. I personally preferred that and I thought that option might be useful for other scenarios as well. I have a proposal for a Postorius adjustment ready as well in case you agree that this would be a suitable approach.