Mailbox support for customer replies and company replies akin to `expenses` mailbox; bidirectional replies to email notifications ala GitHub notifications for users and clients/customers

The likes of Microsoft Dynamics and Wix allow a company user to send out emails to the customer (which we can do already, of course, but they're templated), so it would be great if the company and the client/customer could have conversations that are centralised in Invoice Ninja. This could function similarly to the current expenses mailbox system where external users can email things to a designated mailbox and they end up on the system. Ideally, this should work like GitHub email notifications where a reply to an email notification from a customer will be slurped up into Invoice Ninja and go into an appropriate Communications tab to centralize it all. Each thread or communication regarding a specific item, such as a quote or invoice should be grouped for organizational purposes, and likewise, any general communication not associated with an item. Any replies to these things from the company user on Invoice Ninja should go directly to the customer via email and produce anothe…