The previous systemd hardening prevented the Go networking library from
resolving a hostname to an IP.
Adding the correct permission fixes the hostname resolution, which fixes
the retrieval of link previews by Mattermost's server.
Fix#340
Starting from Mattermost 4.8, the server process sends a systemd
notification when it is ready to accept network connections. Thus we
don't need to explicitely wait for the port to be open in the service.
As a bonus, by removing the hack, we don't need to replace `__PORT__` in
the service anymore–which means the associated script helper can be
removed too.
