Improve context/background for Signal verification numbers

[amandabee]

Participants in the Signal session often have some experience with Signal. One question that came up in the advance team workshops that trainers had trouble answering was: what does the verification number mean? There were a lot of questions about how to use verification numbers and what to use them for.

One participant had previously installed Signal and was using it to communicate with a source when their safety number changed -- he had a lot of questions about what that meant and what to do about it. Trainers didn't feel equipped to answer that question, but the lesson plan should include enough background to prep trainers on this.

Let's discuss here before adding it to the lesson, but ultimately it should be added to the lesson.