Internal commenting system for `quotes`, `invoices`, `products` etc with email notifications that support replies ala GitHub notifications

This would take a ton of friction out of multiple users discussing what's actually on Invoice Ninja if every item on the system had a comment system for it, that way we could just @ a certain user to subscribe them to the thread (if we don't set defaults for certain things) and that'll send out an email notification and allow users to respond to it via email as well as on Invoice Ninja itself ala GitHub.