Add validation for filename in alt text

Problem/Motivation

Including the filename in alt text is a bad practice. We need to optionally prevent that from happening.

acceptance criteria

• Consult UX team on the content of the error message
• If enabled, validates that the alt text is not the same as the case insensitive filename including the file extension
• trims the alt text before validating

Blocked by ✨ Add optional check for filename to admin form Active