User Options:

• Introduce bbp-user-options.php to handle all user option related code.
• Add $user_options array to main bbPress class to allow for easy, static, per-user option overrides.
• Tweak action order for option and user_option overrides in bbp-core-hooks.php.
• Replace all appropriate user_meta calls with user_option calls to automatically prefix keys with blog ID.
• Update favorites, subscriptions, and topic/reply count option keys to be per-blog in multisite configurations.
• Fixes #1826.