Add UI option for Squid workers configuration directive

[danieljloughlin]

Allows the squid workers configuration directive to be set via the UI, and include the directive in the Web Safety backup/restore process.

Current workaround is to manully edit /opt/websafety-ui/var/console/squid/templates/squid/conf/smp.conf

The configuration directive is important for scaling Web Safety in large environments, especially with SSL decryption enabled.

Squid documentation: http://www.squid-cache.org/Doc/config/workers/