Navigation - Move global navigation links to top bar

The global navigation will be moving out of the dropdown where it currently lives and will be laid out in the top bar.

Projects, Groups and Activity are the most common Dashboard pages, so they are always displayed. Clicking on any of these links takes the user directly to the corresponding dashboard page.

Milestones and Snippets are less popular, so they are displayed inside a dropdown

Resources

Specs

Meta issue #32794 (closed) - Global and Contextual Navigation