Collapse merge request sidebar for changes only on smaller screens

I think merge request sidebar should collapse on “Changes” tab only when it can increase size of “Changes” tab content

[11:54] 
otherwise it useless re-center of content

[11:55] 
so it should be a case for screens that are   < 1600px

[11:55] 
for full hd screen it does make sense as it does not increase content size anyway

cc @jschatz1 @rspeicher