CSRF token for decryption

[reecebrowne]

Description

Adds CSRF token to decryption call

Checklist

• [ X] I have read the Contribution Guidelines
• [ X] I have performed a self-review of my own code
• [ X] I have attached images of the change if it is UI based
• [ X] I have commented my code, particularly in hard-to-understand areas
• [ X] If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• [ X] My changes generate no new warnings
• [ X] I have read the section Add New Translation Tags (for new translation tags only)