UX - possibility to inform where the unique value is already being used

Problem/Motivation

While its not always something you might want to show, if where the non-unique value is being used is not private, it might be interesting for usability purposes to show where the non-valid element is being used. The error might not be in the entity which we are currently modifying.

Steps to reproduce

Proposed resolution

Create a token in the "Unique message validation" field which allows to show a link to the violating entity. Doing this allows the web administrator to choose if this value can be shared or not. `unique_content_field_validation_field_is_unique()` could, instead of returning a bool, return the array it finds and let the parent code decide if it wants to inject the entity link or not into the error message.

Remaining tasks

User interface changes

API changes

Data model changes