Refactor concierge

If we are not going to use Concierge as a class, it should become a module. 
Now the syntax is shorter and more clear
Actually I'll allow this. The method is super simple and it doesn't seem like we'll need to keep track of any state for something like this anytime soon.

But, this is probably what I would like best. It does a few things for us. Allows us to test each tiny method in isolation, gives us an easy way to extract future logic into smaller methods that have access to instance variables (er well methods in this case), and still provides a "functional" interface for use elsewhere in the app. Might be overkill for this case, but this style seems to lend itself really well to apps that are still heavily under development.

```ruby
class Concierge
  def self.find_conversation(account, params)
    new(account, params).find_conversation
  end

  attr_accessor :account, :params

  def initialize(account, params)
    self.account = account
    self.params = params
  end

  def find_conversation
    conversation_by_number ||
      conversation_by_recipient ||
      create_conversation
  end

  def conversation_by_number
    account_conversations.find_by(number: params[:conversation])
  end

  def conversation_by_recipient
    Conversation.match_mailbox(params[:recipient])
  end

  def create_conversation
    conversations.create(subject: params[:subject])
  end
end
```

Just my 2 cents.