New job for dropping studies, improved progress tracking

This release includes a new job that allows to drop a study. Users only need to provide the study ID and the job will take care of the variants and the files where they were reported.

Progress tracking has been improved in the step that loads variants into the system. The completion percentage is now displayed, along with the number of variants read from the VCF, written into the database, and skipped due to any kind of issue. This tracking will be added to other steps in future releases.