Rework accept email subject to avoid confusion over an actual accepted mail from the official archive

Closes: #119 (closed)

Using:

hello-1.1: ACCEPTED on mentors (unstable)
hello-1.1~bpo1: ACCEPTED on mentors (buster-backports)

Instead of:

hello-1.1: ACCEPTED into unstable
hello-1.1~bpo1: ACCEPTED into buster-backports

Tested locally.