mediawiki.ui: checkbox: Fix gap between bevel and border

Use an inset box-shadow instead of adding another pseudo
element. This removes the gap and also keeps the look
consistent on different zoom levels.

Change-Id: Ibf75a09ef9955a5d1cb9d80bff2cce001b60d7c6
Merge "mediawiki.ui: checkbox: Fix gap between bevel and border"