Top level new nav bar should be responsive to width changes

For viewports with a wide enough width, we shouldn't show the "More". Instead, show all global links by default and collapse them into a “More” dropdown as space gets tighter:

Current Proposal
Screen_Shot_2017-07-13_at_10.30.13_PM after

Design proposal and specs from https://gitlab.com/gitlab-org/gitlab-ce/issues/35424

Edited by Pedro Moreira da Silva