Twenty Fifteen: Add space between the header and the secondary for tablet size.

[iamtakashi]

I've been on the fence about it but I'd like to give a space to detach the secondary from the header. Initially the secondary was designed as an extension of the header but since we don't apply custom header image and color to the secondary on that layout they should be separated.

Current:

After:

[iamtakashi]

Add space between the header and the secondary

[iandstewart]

In 30243:

Twenty Fifteen: "detach" widgets from the header in mobile views.

Props iamtakashi, fixes #30252.