Theme Compat:

• Put back 'bp_register_theme_directory' sub action, hooked to 'bp_loaded'
• Move register_theme_directory() back into BuddyPress root class method.
• Introduces old_themes_dir variable, which will get deprecated when we remove bp-default from core.
• Create 'bp-templates' root folder, for theme-compat templates to live in, and not conflict with 'bp-themes'.
• Fixes issue where 'bp-legacy' would show up as "Broken" in Appearance > Themes.
• Brute-force switch from bp-default to WP_DEFAULT_THEME on deactivation, and update theme roots, to fix issue when deactivating BuddyPress while theme stack relies on bp-default.
• See #3741