Masterbar: Refactor and isolate logged out functionality

[ehg]

At the moment, the Masterbar is coupled heavily to logged-in dependencies and components. This makes it both hard to reason about, and hard to server side render.

Let’s refactor the Masterbar, so it:

• Is decoupled from logged-in and logged-out scenarios
• Is rid of unnecessary subcomponents
• Reads better

We should get #631 in before we start.

@mtias — do you have any refactoring tasks to add here?

[ehg]

Noting we should ES6ify and normalize the name of the StatsItem, introduced in #1894

[ockham]

Closed by #1918+#2036