Add layout for Content Object Store

[pezholio]

Background

When we began building the Content Object Store, we decided to build it in Whitehall, because we didn't want to maintain multiple apps, repos and deployment pipelines. As we roll out the project, our ambition is to have it linked from signon (in a similar way to the Homepage editor in Mainstream for two reasons:

1. Users should be able to select what action they want to take directly from signon, rather than understand what app they need to be in first; and
2. The navigation in Whitehall is already quite overloaded, so adding another item to the "Other" page risks causing more confusion.

This PR

This alters the layout for the Content Object Store so it appears, to all intents and purposes, as a separate app. I've made some minimal changes to the existing layout, so we don't have to repeat ourselves unnecessarily, only specifying the app name and a custom navigation. Other elements, such as the footer etc, should be easy to override too. Conversely, if we decide after user testing that this is not the way to go, we can easily revert it.

Screenshots

[pezholio]

That's a really good point! I think I'll get this out of draft anyway, but it's a discussion I'll look to open up in the coming weeks. It certainly seems like we're starting to outgrow Whitehall already, and things are only going to get bigger!