Refactor permission checker for clarity

[oqkr]

The current permission system works, but it's a little hard for me to reason about without leaving detailed comments to myself (see isAllowed function).

This probably needs to be refactored for clarity/simplicity or reworked.

It's not so much that the system is complex as that parts of it don't behave in a straightforward manner — like the fact that an empty allowedUsers field means "allow everyone" in a direct-message context yet means exactly the opposite ("ignore this and use allowedRoles") in a guild-channel context.

It's been suggested that a better route may be to treat permission checks more like a chain and at each part of the chain make a decision to allow, deny, or defer to the next link in the chain using something like this pseudocode:

/** pseudocode */
if (isDM) {
  permissionsChain = [
    new EveryoneCheck(command.allowEveryone),
    new AllowedUsersCheck(command.allowedUsers) ]
} else if (isGuildMessage) {
  permissionsChain = [
    new EveryoneCheck(command.allowEveryone),
    new AllowedRolesCheck(command.allowedRoles),
    new AllowedUsersCheck(command.allowedUsers) ]
}
if (permissionsChain.allow()) {
  // do stuff
}

But for now, it does actually work, so this is a low-priority item that I'm just noting for later.