Move roundcube out of webroot

[senilio]

Keeping it directly in the webroot causes issues when fronting with a reverse proxy running in a subdir. Running roundcube from /var/www/html/webmail will make it more proxy friendly.

[kaiyou]

I do not see any idea why not. It would simply things with external reverse proxies and even make it possible to host other content in other subdirectories. I will also have the front nginx serve the Webmail as /webmail and redirect / to /webmail.