Navs: add toggle button for expanding/collapsing the ToC and the global nav

• For https://gitlab.com/gitlab-com/gitlab-docs/issues/251 and https://gitlab.com/gitlab-com/gitlab-docs/issues/298

For large screens:

• Add a toggle button for the global nav (on the left)
• Add a toggle button for the ToC (on the right)

For mobile:

• Add a toggle button for the global nav (left), currently hidden on @media(max-width:1099px)