[Completes #50370923] Added 'Update your notification preferences' to ac...

Please see https://www.pivotaltracker.com/story/show/50370923 for background.

Added "Update notification settings to account settings".
Created migration to add notify_me attribute to user and update past user notify_me values to true.
Added edit_notification_settings and update_notification_settings to users_controller
Set notify_me to true on before User create.
Added form with checkbox for users to toggle receive notifications setting.