Gather success metrics on VeraCrypt integration

• Evolution of the number of WhisperBack reports on using
  VeraCrypt volumes (absolute number and fraction of all WhisperBack
  reports) measured three times over the first three months after
  the beta release.

• Portion of our users who report using VeraCrypt volumes. This
  figure will be compared to the target user base identified in A.1.

Parent Task: #14468

Subtasks

• #15977 (closed)

Original created by @sajolida on 15976 (Redmine)