Use `bool` for testing non-emptyness of address fields

Thanks @fatiherikli
Render promotions within content block of layout.html

Promotions were omitted from the main layout block (accidentally I
imagine).  I think they used to be there - perhaps a template refactor
stripped them out.

This commit reinstates them so pages that use the default layout
template (like flatpages) can have promotions added to them.