[Feature request] Invoice tracking

[josemi-ca]

It would be useful if it a web beacon was automatically included so that they are marked as "viewed" when the clients open the email. This could be a "viewed" checkbox similar to the "sent by email" one.

[ParisLiakos]

This needs a tracker 1x1 img in the email most likely. So, when the user opens the email, this image is triggered. The image is just a script though which updates the DB for this invoice to "opened/viewed"