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-fetchers.js

Existing code: Fetches the default_role when constructed threadInfos to set the isDefault flag for each role in threadInfo.roles

Depends on D9979

Addresses part of ENG-5833

Updated my query and logged the threadInfo.roles. Confirmed that for each role, isDefault was set to true for all Members and false otherwise.

Another test I did was to compare the logged threadInfo.roles both before and after the query change and they were identical