Problem Description: 

Currently upload signature button appears using `GALAXY_REQUIRE_SIGNATURE_FOR_APPROVAL` which is ok for CRC

However for testing environments, standalone deployments and on-prem hub it would be nice to have a specific setting flag.

Proposed Solution: 

Use `PULP_GALAXY_SIGNATURE_UPLOAD_ENABLED` to control the appearance of the upload signature buttons on approval dashboard and also on collection detail pages.

This process is already documented: https://galaxyng.netlify.app/config/collection_signing/#signature-upload