Bug: `Bot.get_guild_key()` doesn't return default value

If Bot.get_guild_key() is called for a guild configuration key that isn't set, it will return None. This is due to it only querying Bot.config_data rather than initializing GuildConfig to query the DB if needed.

This ought to be fixed. Presently, this bug affects entrant scanning. The default scan policy cannot be retrieved, cause none of them to be ran if this feature of member verification is enabled.