todo.sr.ht/#118: Comment notifications send via mail should indicate who send them

Now that mails are not sent on behalf of user's e-mail account, outgoing@sr.ht is too generic to be used in From: field.

it would be good to use something more informative, like:

"username" <username+comment@sr.ht>

It's a bit quiet in here.

~sircmpwn/todo.sr.ht/#118: Comment notifications send via mail should indicate who send them