Reserve space in UI for search result counts

Summary

The search results page adds badge counts to the results tabs. Until recently, the page got loaded when this was already calculated, but with #13326, this now happens asynchronously, causing the UI to jump multiple times as these calculations finish at different times.

Possible fixes

We could let the badge count already be present and add some loading icon to it, but the space these badge counts need can vary from 1 to 1001+, which would still cause the UI to jump. There are a couple of other ways to go, that might even also solve how little prominent the tabs are, causing people to expect there were no results for them even though that's only the case for the first tab.