fix(case-studies): map _caseStudy field to first title (#234)

- Updated _caseStudy schema mapping;
- Ensures the frontend now correctly displays the title of the first
related case study;
- Fixes API errors when saving drafts related to _caseStudy.

Author: IhorMasechko

website/modules/testimonials/index.js

@@ -106,6 +106,10 @@ module.exports = {
         label: 'Organization',
         name: 'organization',
       },
+      _caseStudy: {
+        label: 'Related Case Study',
+        name: '_caseStudy.0.title',
+      },
     },
   },
   filters: {