Add a global "repositories are cloning"/"Sourcegraph is not ready yet" indicator

Created by: mrnugget

As discussed in Slack, I'm quoting myself here:

I wish we had a global "repositories are cloning"/"sourcegraph is not ready yet" indicator. After setting up a new Sourcegraph instance and adding your GitHub repositories (which can easily be in multiple hundreds if you use affiliated as repositoryQuery) it's pretty disheartening that you don't get search results for things you know have to be there. And if you're a new user you don't know that you can check "Repositories"

Also including @sqs's reply:

the fact that some repos are unavailable is shown very diminutively in the search results header, but basically everyone misses that. So we should remove that and treat "repos currently being cloned" as a global notice, not a per-search-results-page notice. (This makes sense because it would also affect code browsing and cross-references, not just search.)

I also found that this ticket is highly related to:

• https://github.com/sourcegraph/sourcegraph/issues/3063
• https://github.com/sourcegraph/sourcegraph/issues/4120