[finishes #70394414] add minimal-ui viewport tag for iOS 7.1

https://github.com/code-dot-org/blockly/pull/530 is related

This adds the minimal-ui viewport tag for iOS 7.1+

Note that it must be in the tag at load time (the dashboard initially creates this viewport tag) in order for it to work properly. The blockly code that modifies the viewport tag (in the other PR) includes "minimal-ui" to preserve the value, but that code alone is not sufficient since it modifies the tag after the page has started loading.