Fix to ptif image conversion and adding a date-based bulk convert task

[ajolipa]

In this PR

• Updates the to_tiff conversion function along the lines suggested in Issue PTIF output is missing pyramid levels on ImageMagick builds from mid-2023 to July 2026 #123 ;
• Adds a bulk conversion rake task that accepts start and end dates as parameters, for use in re-converting existing images that were processed during the affected time range for the PTIF bug.

[blms]

Looking great! Just one suggested change.

[ajolipa]

Looking great! Just one suggested change.

Suggested change makes sense to me; but then would that reasoning also apply to the other tasks in this file, or at least some of them? (convert_images, convert_images_by_type, convert_images_by_colorspace) Although I suppose those were probably written with the intent to include images that had never yet been converted, whereas in this particular context of fixing the flat TIFFs we only need to target resources that have converted_content? Still the inconsistency between otherwise basically identical tasks feels a bit weird.