KAZOO-2238 voicemail app prompt customizations

Added two new voicemail box properties to enable the new behaviors.  If not set, or set to false, the previously existing behavior is the default behavior.

play_greeting_intro - if set to true, play vm-greeting_intro.wav prompt prior to playing the default or user-provided greeting prompt.

use_person_not_available - if set to true, play the vm-person_not_available.wav prompt (rather than the current vm-person, <box number>, vm-not_available prompts) when playing the greeting for a box that does not have a user-provided greeting prompt.

I provided two placeholder audio files.  You guys will probably want to generate better ones where the voice matches the other prompts.  I couldn't figure out how to generate a prompt with a matching voice.