[lib] Use PERSONAL instead of GENESIS_PERSONAL for default thread noun in getNameForThreadEntity

Summary:
This diff is a no-op, as both of these return the same thing when passed to threadNoun. I thought it would be better to use PERSONAL here, to reduce the number of places in the codebase we use GENESIS_PERSONAL.

Depends on D13551

Test Plan: Careful code inspection

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13552