Hide the branding icon in devise and static page views, for xs- and smaller viewports.

It took too much horizontal space in small smartphones, pushing the page header into two lines.
Less wasted space in the landing page, in smarpthone screens.
Less wasted space in the footer for the landing page, in small smartphones.
Less wasted vertical space in landing page, in small smartphones.
Less wasted space in devise views, for small smartphones.