snappymail: fix nginx root and /snappymail prefix handling

[Infern1]

Changes proposed in this pull request:

• snappymail: redirect /snappymail to /snappymail/ so the bare path doesn't fall through to nginx's default root
• snappymail: correct document root — haproxy does not strip the /snappymail prefix (unlike roundcube and rainloop), so root must be /usr/local/www
• snappymail: scope the data deny rule to /snappymail/data, which is the URI that actually arrives

With root /usr/local/www, both paths resolve: /snappymail/ →
/usr/local/www/snappymail/index.php, and assets built from
app_path = "/snappymail" → /snappymail/snappymail/v/<ver>/static/... →
/usr/local/www/snappymail/snappymail/v/....

Note for existing installs: store_config() won't overwrite an existing
snappymail.conf, so this only helps fresh deploys. roundcube.sh handles
that case with migrate_roundcube_nginx_conf() — happy to add an equivalent
here if you want it, though moving people's configs felt like your call.

Fixes #689

Checklist:

• docs up-to-date

[msimerson]

I can't replicate the issue on an upgrade nor on a completely new build. Without these changes, the install works just fine. Even if it hadn't, changing the root isn't the right way to fix it (rewriting the path in nginx would be better).

[Infern1]

So I never deployed snappymail, switch to using it, since rainloop is not maintained.
Did a git pull and provisioned snappymail. Running 14.4 FreeBSD. Latest haproxy config

Yesterday I updated to base p8

service jail stop snappymail
Stopping jails: snappymail.
#zfs destroy -f  zroot/jails/snappymail
#zfs destroy -f  zroot/data/snappymail

The gist
https://gist.github.com/Infern1/f87f0505489f70d59521b5ab1545b1d2

And now I retried and it is working, really strange