SSBC: See "overview" from execution page when no workspace is selected

Created by: courier-new

Currently the execution page doesn't offer much to look at until you drill into a specific workspace and just says "Select a workspace to view details." We could do more with this space to give a better overview of the execution progress.

For example, particularly at the beginning of execution when we might be waiting in the queue for a while, it might help to surface the earliest position in the queue:

Or, once we start to have changesets available to look at, show "recent changesets" here:

(We've talked about these before and may have even started designs for them but I can't find them at the moment)