web: Add StatusMessageItem to NavBar

Created by: mrnugget

This is the second part of the implementation of #4120, the UI, as described in https://github.com/sourcegraph/sourcegraph/issues/4120#issuecomment-499911568. (It's a stacked PR that depends on #4440 being merged first)

This adds the StatusMessagesNavItem to the global NavBar. Read through #4120 to get a general idea of the feature and what this MVP entails.

Here is what it looks like when nothing is currently being cloned/updated:

Here is what it currently looks like when repositories are being cloned (or updated/fetched!) in the background:

What I'm still unsure about:

1. Maybe it makes sense to add a header to the DropdownMenu? The goal for this item is to be expanded in the future and a header is a chance to give more context, like this maybe?

2. The wording, here and in #4440. Since the spinner activates whenever we do the initial clone of **OR** just update a repository, maybe "cloning" is wrong and we should change it to "updating"? Technically it is cloning, but it happens far more often than the initial cloning

Reviewers: I'm totally new to the web codebase and its conventions, so, fire away :)

Test plan: jest tests