Updates to webmail

Our webmail installation has been updated to the latest version of the underlying software (the 1.4 series of Roundcube). Amongst other improvements, this includes a new theme, designed responsively for all sizes of screen, and so should be easier to use upon a mobile device.

At the same time, our (very) old alternative webmail, Squirrelmail, has been removed. Roundcube has over the past several years become superior in every way, and usage of Squirrelmail had dropped to effective zero. Removing unused software allows us to redeploy resources that would have been used on maintenance on other improvements.