New Webmail interface test

We are trialling a new webmail interface that we'd like to roll out as an alternative to Shinymail and Squirrelmail. Called Roundcubemail, it's faster than Shiny, slicker than shiny or squirrel, but doesn't quite have as many features as Shinymail.

Feel free to test it and let us know what you think of it.

If successful we'll put a link to it on the Shinymail webpage.