Export announce subscriber list

Created by: temparus

The announce subscriber list needs to be accessible from the mailing list server over HTTP protected with basic authentication.

There are two possible solutions:

1. Transfer the list to a samba share (or sshfs mount) running a web server on a regular basis.
2. Add another endpoint to the API serving the list of announce subscribers. This endpoint needs to be protected using HTTP basic authentication.

Opinions? @cburchert @NotSpecial @hermannsblum