By Julia Banim You’ve heard of school uniforms and work uniforms, but have you ever heard of a ‘Sunday uniform’? For most of us, getting dressed on a Sunday morning usually involves getting as comfy as possible – while trying to banish all thoughts of Monday – but M&S has decidedly different ideas. In a Facebook post that has since attracted some pretty brutal mocking, the iconic British retailer appeared to suggest the go-to Sunday look would involve kicking about in a pair of bikini knickers and a midriff-bearing shoulderless top. Now, for those of us with shopping lists to tick off, relativ…