Closes #8198: Custom field uniqueness (#16661)

* Closes #8198: Implement ability to enforce custom field uniqueness

* Add missing form fields & table columns for validation attributes

* Remove obsolete code

docs/models/extras/customfield.md

@@ -107,3 +107,7 @@ For numeric custom fields only. The maximum valid value (optional).
 ### Validation Regex
 
 For string-based custom fields only. A regular expression used to validate the field's value (optional).
+
+### Uniqueness Validation
+
+If enabled, each object must have a unique value set for this custom field (per object type).