Add gromox-cleanup.sh script

[deajan]

Purpose of this PR: Keep gromox folders clean

While playing with gromox, I wrote a prometheus exporter here after which I noticed that some mailboxes were quite huge, even when the ondisk maidir was twice as small.

Discussing with @WalterHof and @crpb, they suggested to run various cleanups.
I was wondering why that wasn't "standard" for Grommunio, so here's a script that perhaps should be added by default to Grommunio's setup via a systemd timer / or even as fs-hook/thaw for VM environments.

Currently tested on two Grommunio servers running gromox 2.37.

Thanks for considering this.

[WalterHof]

Do not write to a log file (/var/log/${SCRIPT_NAME}.log). Write to systemd logging instead,

[crpb]

Write to systemd logging instead,

ahh right. and with the .service you can just echo you to stdout and it will show up in the journal under --unit grommunio-cleanup or however it might be called in the end.
And for different levels logger/systemd-cat -p err/info/...

[deajan]

Pull request updated as per your suggestions. New script has been tested again.
@crpb Note that I have changed the timer to make it run everyday at 2am.

There's just one thing, I don't know where @jengelh would place the script, so I cannot convert the systemd unit file to a template file.