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 `role-fetchers.js`
Existing code: We determine whether or not a role is a default role based on r.id = t.default_role to then return in an object.
Depends on D9975
Addresses part of [[ https://linear.app/comm/issue/ENG-5833/update-code-that-uses-default-role-from-mariadb | ENG-5833 ]]