Improve UX of left sidebar (navigation)

Based on user testing we found that left sidebar is confusing (changing context, people don't understand that it has hierarchy...) To improve usability of application we decided to redesign context navigation and keep left sidebar static.

TODO:

• Decide on concept - https://gitlab.com/gitlab-org/gitlab-ce/issues/14838#note_5083863
• Desktop horizontal navigation for profile pages - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3824
• Desktop horizontal navigation for group pages - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3980
• User testing of profile and group navigation - https://gitlab.com/gitlab-org/gitlab-ce/issues/17266
• Make left sidebar a bit smaller to have more width for content (from 230px to 220px) - https://gitlab.com/gitlab-org/gitlab-ce/issues/17338
• Mobile improvements for new horizontal navigation
• Hide left sidebar for mobile screens - https://gitlab.com/gitlab-org/gitlab-ce/issues/17323
• Desktop+mobile navigation for project pages - https://gitlab.com/gitlab-org/gitlab-ce/issues/17522
• Add new icons to sidebar - https://gitlab.com/gitlab-org/gitlab-ce/issues/17579
• Compact sidebar - https://gitlab.com/gitlab-org/gitlab-ce/issues/18013
• Admin area navigation - https://gitlab.com/gitlab-org/gitlab-ce/issues/18338
• Reduce amount of tabs in horizontal navigation - https://gitlab.com/gitlab-org/gitlab-ce/issues/17983
• Hide sidebar by default https://gitlab.com/gitlab-org/gitlab-ce/issues/18417