Add an 'OCR' filter to the digitisations 'Requests report'

Currently, we upload all digitisations with OCR (Optical Character Recognition) applied to support accessibility.

Within each digitisation, there is a function to detect if the scan has OCR or not.

It would be useful if there was also an 'OCR' filter in the Requests report in TADC.

This would help us to easily check that all the digitisations have OCR, and/or if any have OCR missing due to human error or predating this policy.