Implemented a 'web root' config/constant, related to #34

Now, the $config['web.root'] value is available in the Twig templates via the variable WEBROOT.
Wherever we need to 'redirect to root' (e.g. the SSHCure logo), or need to construct a path/url in the front-end, we should create it using either $config['web.root'] in PHP or WEBROOT in Twig/HTML.