chore(organizations): add search field to organization user admin page (

#5452)

### 📣 Summary
Added a search field to the organization user admin page to simplify
user lookup and enable narrowing down the export list.

### 📖 Description
A search field has been added to the organization user admin page,
allowing administrators to efficiently find organization users by
username or organization name and id. Additionally, this search
functionality can be used to narrow down the export list, making it
easier to export only the desired subset of users.

kobo/apps/organizations/admin.py

@@ -53,6 +53,7 @@ class Meta:
 @admin.register(OrganizationUser)
 class OrgUserAdmin(ImportExportModelAdmin, BaseOrganizationUserAdmin):
     resource_classes = [OrgUserResource]
+    search_fields = ['user__username', 'organization__name', 'organization__id']
 
     actions = (
         create_export_job_action,