batches: show total count in workspaces preview header
Created by: courier-new
Closes https://github.com/sourcegraph/sourcegraph/issues/34102.
Adds "Displaying X of Y" to header for workspaces preview. Demo with page size of 5:
https://user-images.githubusercontent.com/8942601/173506483-08a2fb20-4a10-471b-a17d-ace58dad8404.mov
Idk why the whole preview kinda flashes right when the first preview resolution succeeds, but that's a different issue.
Test plan
Manually tested, as in the video recording above, with a page size of 5 and multiple states of the workspaces preview resolution.
App preview:
Check out the client app preview documentation to learn more.