Rework submit when called by host

[zig1000]

Currently, !tournament-submit when called by the host allows them to submit for other players in case of special circumstances. This is desirable. However, it also allows for submission with non-registered nicknames. This was intended to allow the host to submit for non-discord-using players, but has never been needed and is more trouble than it's worth, as it forces the bot's data model to support solutions with nicknames not in participants.json (which is keyed on discord tags). This can lead to unexpected bugs, e.g. the host command !tournament-submission-delete currently breaks on such solutions.

Remove the ability to have non-discord-associated submissions/nicknames, and rework submit when called by the host so that it allows for submissions on the behalf of other players only if they are already registered in participants.json, and behaves like a regular player submit from the host if the nickname is already registered to them or they aren't registered (possibly also warning the host that hosts generally shouldn't participate on first-time registration).