Restore Add Friend admin page

[akirk]

Summary

• The Add Friend page (admin.php?page=add-friend) was accidentally removed in Remove Friendship functionality #515 (Remove Friendship functionality), while the intent was to move it to the frontend (Add a Friend on the Frontend #516 — still pending)
• All existing links (admin bar, bookmarklets, welcome page, blocks, notifications) still point to this page, causing an "access denied" error when users try to follow someone from a boosted post or elsewhere

Fixes #619

Changes

• Re-registers the add-friend submenu page
• Restores render_admin_add_friend(), process_admin_add_friend(), and process_admin_add_friend_response() — simplified to subscription-only (no friendship request, codeword, message, or role selection)
• Restores templates/admin/select-feeds.php (feed selection step 2) without friendship UI
• Restores templates/admin/latest-friends.php (recent subscriptions table)

Test plan

• Visit admin.php?page=add-friend — should show the Add Friend form instead of access denied
• Enter a URL and click Next — should discover feeds and show selection
• Use Quick Subscribe — should create subscription directly
• Click Follow on a boosted post — should navigate to Add Friend page with URL pre-filled

Test in WordPress Playground