In Vector a "Vector preferences" section appears when the "vector" skin radio button is selected (see https://en.wikipedia.beta.wmflabs.org/wiki/Special:Preferences#mw-prefsection-rendering)

In Monobook a Monobook specific preference (responsive mode) only shows when the user hits save.

This task aims to make the two workflows consistent with a core change that also resolves T246162

QA steps

• Make sure Vector skin is selected
• Visit Special:preferences
• Click Monobook

Expected: I see the Monobook preference.
Actual: I don't see it until I hit save.

Acceptance criteria

• The Monobook specific option is revealed as soon as you select the radio button. When another radio button is selected it is hidden.
• The Monobook specific option appears under a heading "Monobook skin preferences" when you select the radio button.
• When selecting the Vector skin a Vector specific option appears under a heading "Vector skin preferences". The Monobook skin is no longer shown
• The skin preference section appears directly under the skins and is the first section in preferences.