Gather success metrics on Additional Software

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

• Portion of the existing user base contacting us through
  WhisperBack who uses additional software now that it has a
  graphical interface (independently from whether they are reporting
  a bug on this feature or not). This figure will be compared to the
  baseline identified before developing the alpha version.

Parent Task: #14568 (closed)

Original created by @sajolida on 15979 (Redmine)