Allow for separate per-project navigation

As part of improving the work on the global navigation, it's important that separate projects are able to have their own global navigation.

Currently, the following projects take whatever navigation CE/EE provides them:

• Runner: https://docs.gitlab.com/runner/
• Omnibus: https://docs.gitlab.com/omnibus/

See #298 (comment 192570353) for some discussion.

See global navigation documentation: https://docs.gitlab.com/ee/development/documentation/site_architecture/global_nav.html

Relates to: #298 (closed)