Twitter theme: add second widget area in right column

[akirk]

Summary

• Registers a new "Friends Sidebar 2" widget area (friends-sidebar-2) alongside the existing friends-sidebar.
• Renders it in the Twitter theme's right column between the search box and the customize link, styled as a Twitter-style rounded card that hides when empty.
• Points the right column's "customize sidebar" link at the new widget area (the left column still links to friends-sidebar).

Gives users a place to drop widgets (trends, follow suggestions, etc.) in the previously-empty right column of the Twitter theme.

Test plan

• Switch to the Twitter theme on the Friends page.
• Open Appearance → Widgets (or the Customizer) and confirm "Friends Sidebar 2 (Twitter theme)" is listed.
• Add a widget to it and verify it appears in the right column, styled in the rounded card container.
• With no widgets added, confirm the container is hidden (no empty card).
• Click "customize sidebar" in the right column — verify the customizer opens focused on the new widget area.
• Confirm the left column's "customize sidebar" still points at the original friends-sidebar.

Test in WordPress Playground