wp_is_mobile: maybe differentiate between tablets and phones

[iandstewart]

Maybe we should differentiate between tablets and phones. For example in Twenty Fourteen we hide content on phone-sized screens with display none which falsely suggests the content is not loaded -- and it probably shouldn't be. We can't use wp_is_mobile in it's current state to hide it because it doesn't differentiate between tablets and phones. Twenty Fourteen, for example, _does_ show content on tablets even though it doesn't on phones.

The attached patch is a proposal for allowing us to take care of that.

[dimadin]

wp_is_mobile() shouldn't be used on frontend since it doesn't work when page caching is used. I opened #26221 to address this issue.

[johnbillion]

Duplicate of #24541.