Allow sending of messages together with payments

[akarelas]

In some websites that accept donations I'm pseudonymous and wish to tell the website owner what my username on their website is, or which feature/bug I wish a developer to focus on.

Of course, the owner should be able to block messages from me, to avoid spam and abuse. But I think the payer needs to know whether he's blocked, so he'll avoid writing long texts that will never be read.

[Changaco]

Closing as a duplicate of #771.