dev: mocktwilio UI

[mastercactapus]

Split from #2554

What problem would you like to solve? Please describe:
To develop the SMS and/or voice functionality of GoAlert, you need a Twilio account and publicly-routable access to your development machine.

This is difficult in enterprises that may wish to contribute, or for developers to test/evaluate the project.

Describe the solution you'd like:
Add a developer UI to the mocktwilio server that can emulate the basics of a mobile device, concerning SMS messages and phone calls.

As a +1, adding this to the dev server to test interactivity could be considered.

Describe alternatives you've considered:
Using tools like ngrok or sendit (from the repo) and having engineers each with their own dev number.

Additional context:

Groundwork started here: #2602

[stale]

This issue has been automatically marked as stale because it has not had recent activity.