Better (symmetric) padding for sidebar main area

[cpsievert]

Closes #838

This PR makes some small tweaks to layout_sidebar()'s main content padding when the sidebar is collapsed. Specifically:

• On desktop, increases right padding (of the main content area) to match the left padding
• On mobile, increases top padding (of the main content area) and decreases left padding (to match the right padding)

Here's a minimal example to visualize the difference:

page_sidebar(sidebar = sidebar("Sidebar", open = FALSE), card("A card"))

Before (desktop)

After (desktop)

Before (mobile)

After (mobile)

[gadenbuie]

Looks good! I like the mobile layout and I was pleasantly surprised by the desktop layout version.