Allow customisation of "Board index"

OK, so in phpBB 3.1, it's now possible to have a home button that points to another link (either on the same site or a different one). For those who use the forums with their site, I think it would be helpful to be able to change the name "Board index" to something of the user's choice. For example, phpBB.com's forums are under /community, so when 3.1 goes on the website, the link could look like this: phpBB.com < Community < ... breadcrumbs as needed, etc, etc.... Obviously, if the user doesn't decide to change the name, it could fall back on "Board index".

This should be simple to add before 3.1 gets pushed out - just add a config var to the SQL, and ACP field, and then add template vars to to change the name accordingly.

RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=44085