Performance: Reduce layout shifts in search results

[umpox]

Description

Goal: Improve perceived performance when navigating through search results by reducing CLS
Areas to fix:

• Selecting search filters: “Show more matches on all results” button to pops in and out
• Submitting search: Loading result count -> 0 results in 0.00s -> 5 results in 1s shifts
• Submitting search: Dynamic filters and repositories pop-in suddenly in sidebar
• Selecting dynamic filters and repositories cause themselves to pop-out and back in

Note: This might be affected by future search/file UX changes

A/C

• All the mentioned layout shifts are fixed!

[sourcegraph-bot-2]

Heads up @taylorsperry @jasongornall - the "team/frontend-platform" label was applied to this issue.