#6 Temporarly include ModernBB 3 installer

We're going to rewrite this completely, but we include this already just
to make it possible to install Luna, not that this is 100% the same
database installer as seen in ModernBB 3 and we will work on this first
before moving on. This is not the final database structure, so there is
no rush to install it already with this script (not that the install.php
page is going to load correctly anyway...).