We need to get rid of usages of threads.default_role throughout the codebase so we can eventually drop the column.
This diff handles the usage in thread-creator.js
Existing code: When creating a thread, the roles from createInitialRolesForNewThread are used to populate the role ID into threads.default_role for that specific row. We should no longer populate this field.
Depends on D9980
Addresses part of ENG-5833