[FEAT] Optional webmail installation

[stdevPavelmc]

Is your feature request related to a problem? Please describe. Some users (sysadmins) requested this option a time ago.

Describe the solution you'd like An option to install a webmail, on the mailserver or in an alternate host.

Most used webmail these days are https://www.rainloop.net/ and https://roundcube.net/

I will start with Rainloop and add Roundcube later on.

[stdevPavelmc]

Rainloop is deplared abandonware, no major commit to the codebase, no new version, etc on a long time.

Moreover, it has an active exploit that leads to remote code execution, so, that's a no go.

We are profiling the active clone: snapy mail.

Maybe we will go in that way, as we can reuse some of the work done.

[stdevPavelmc]

Starting works to add Webmails.

Under this premises:

• As usual defaults are compatible with the previous configs, so no webmails on default, its an opt in feature.
• Snappy Mail or RoundCube, the user pick his poison.
• As much integration with each one as possible (address book, sieve, etc.)
• Nginx as webserver with php-fpm modules and using the same mail ssl certs, only 443, 80 with redirection.
• Same hostname as the mail server, but optional alias for the webmail
• Any other we found under the curse of implementation.