Research Doctrine Indexed Associations to try to ensure tag uniqueness

[gothick]

https://www.doctrine-project.org/projects/doctrine-orm/en/2.13/tutorials/working-with-indexed-associations.html

This was suggested by this Stack Overflow answer to the same question we have: how do we ensure that the "graffiti" tag on one Image is the same tag as a "graffiti" tag on another image.