Extract purgin thumbnail images to a service

Problem/Motivation

At this moment the purging form contains all specific code to make the purging of image pdf thumbnails and if we need this operation to be done in, for example, an update we need to replicate the code.

Steps to reproduce

Proposed resolution

Extract purging code to a service in order to be able to be used on other parts of the code.

Remaining tasks

• Analyse impact of change
• Check what code to refactor
• Create service with the needed code
• Update form PdfImageEntityPurgeForm to call service using dependency injection