Hide 'Select context' placeholder label after first use

Created by: rrhyne

The Select context placeholder helps orient the user to the new feature, but it takes up valuable search bar real estate.

Internal user feedback: "I can't read my search query in most cases with my editor and sourcegraph open side-by-side.."

After the user has activated a context or closed the 'About version context' card, we should also hide the placeholder.

The label which displays a selected version context however, should still display.