Ensure active global nav item is always in view

Description

Inspired by @jerasmus comment: #416 (comment 318269961), implements scrollIntoView() on the active menu item.

• Before: https://docs.gitlab.com/ee/ci/yaml/README.html

• After: http://eread-scroll-menu-item-into-view.178.62.207.141.nip.io/ee/ci/yaml/README.html

• Before: https://docs.gitlab.com/ee/user/project/wiki/index.html

• After: http://eread-scroll-menu-item-into-view.178.62.207.141.nip.io/ee/user/project/wiki/index.html

• Before: https://docs.gitlab.com/ee/api/wikis.html

• After: http://eread-scroll-menu-item-into-view.178.62.207.141.nip.io/ee/api/wikis.html

Related issues and links

Closes #416 (closed). I think this function satisfies the original brief well enough.