[lib] Convert CreateSubThread robotext to use EntityText

Summary:
To make this one work, I had to introduce a new field on ThreadEntity: subchannel

Depends on D6518

Test Plan: Looked at a CreateSubThread robotext on web and native, both from the perspective of the viewer and of somebody else

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6519