Prevent relationship fields from referencing single-type Content Types

[rmartz]

Overview

After encountering a UI issue in which a Relationship that referenced the Record Schema's primary Content Type was rendered incorrectly, it was decided that since each Record could have at most one single-type Content Type, a configuration in which another referenced that Content Type was not meaningful. Fixing the UI issue was deferred in favor of preventing such a configuration from being created.

Checklist

• PR has a descriptive enough title to be useful in changelogs

Demo

Notes

This does not add server-side validation, so invalid Record Schemas can be created by accessing the API directly. This appears to be a currently existing limitation, as we do not enforce other validation such as preventing Content Types from having a Relationship that points to themselves. #164873055 has been created to enforce these restrictions server-side.

Testing Instructions

• Edit a Content Type on a Record Schema
• Add a Relationship field, and attempt to point it to a single-type Content Type like "Incident Details"
  • Such Content Types should not be listed

Closes #164801173