Add capability for admin to bulk SMS all Volunteers

[Redguy1959]

We have an occasional need to send announcements to all of the Helpline Volunteers. We have tried using email but often the addresses are not up-to-date or people don't check their email. SMS would be more reliable.

In the past we have hacked the Twilio code to send information along with any SMS that is being sent from the system to a Volunteer, but this is suboptimal.

[jbraswell]

A lot of service bodies (including my area) just add the volunteers to a WhatsApp group and use it for internal communications.

[Redguy1959]

We are not interested in using WhatsApp for this. It is adding another layer of list maintenance. All of our volunteers are already capable of receiving SMS from Yap. Some of our members do not want to use WhatsApp or are prohibited from using it. We want the requested feature for infrequent one-way communications that will reliably reach all our volunteers without them having to subscribe to anything additional. Sent from my Kyocera DuraXE, an AT&T 4G LTE feature phone