Use template dimensions

We had a missmatch between the template and slide image size dimensions in multiple places in our calculations.
Start work on cleanup tasks

The goal is to create rake tasks that permanently delete no longer used data and clean orphaned slide images from the system. Currently only the clean:images task is implemented
Implement task for cleaning soft-deleted slides.

We find slides in all ThrashGroups and with the deleted flag set and clean them and their associated files.