hide download button on homepage for iOS users bug 767985 

This is a temporary measure to hide the download button on the home page for iOS users, until we have new 'unsupported' text in place.

Update: This PR now also includes some iOS specific styling for the home page firefox-promo banner, to keep the content centered on tablet viewport. This is to make up for the white space created by the absence of the download button content. (Visual layout given the OK by habber on IRC).

This is only temporary styling to be used in the interim.