Introduce Asset-Policy join model

This will enable foreman_openscap to bind policies to hosts.
has_and_belongs_to_many is not that flexible and it does not support
:through parameter.
Rename AssetsPolicy -> AssetPolicy

Join tables are often named in form singular_plural. Interestingly,
the example in 'Active Record Associations' Rails Guide shows
plural_plural in the context of has_and_belongs_to_many.