Scaffold work to implement combined menu

The top-navigation change will be a quite impactful one, so we should be smart about the rollout of the initial work. In this case we aim for the following:

• Feature Flag that allows to render a different version of the top navigation
• Aim for the FF to be enabled only on a per-user basis till we have everything in that's needed
• Think about an option where people within GitLab can self optin and optout of the feature flag, so they won't be blocked by this if something goes wrong.
• Define if there are any other issues needed before kicking off implementations based on the parent epic

Availability and Testing

Since there is a plan to enable this in stages and allow opting in, we should create some automation with the ff enabled state in addition to the default. Please include a SET in MR reviews to keep them updated on the progress. At a later date we might need to adjust our existing tests if ff enabled globally.