Feature/1572 relation field elements matching

[i-just]

Description

When mapping relation fields, for some relation field types, you can choose to map them not only via a predefined list of attributes but also some of the custom fields.

Historically, the list of available match options was a list of all the fields (not just ones that are related to the sections/category group, etc., that are used by the relation field) that were deemed fit to be used as a unique identifier (a selection of the “simple” fields).

With the changes in Craft 5, that has become less intuitive to use.

This PR changes this behaviour so that:

• for the Users field and Author attribute
  • only list the custom fields from the user field layout (there’s only one);
• for the Entries field
  • only list the custom fields from the field layouts attached to the sources used by the field;
  • if only custom sources are selected by the Entries field, show all the custom fields;
  • if there’s a mixture of custom and native sources, list the custom fields from the field layouts attached to the sources used by the field, followed by a list of all the custom fields;
• for the Categories field
  • only list the custom fields from the field layout attached to the category group used by the field;
  • if a custom source is selected for the Categories field, show all the custom fields
• for the Parent attribute for Entries and Categories (when they’re hierarchical)
  • only list the custom fields from the layout linked to the entry type or category group selected when defining the feed

To be clear, in all the above cases, the list of fields is further filtered down to only the ones that can be used as a unique identifier.

Related issues

#1572