In Herald transcript rendering, don't store display labels in keys

Summary:
Ref T13480. Currently, when Herald renders a transcript, it puts display labels into array keys. This is a bad pattern for several reasons, notably that the values must be scalar (so you can't add icons or other markup later) and the values must be unique (which is easily violated because many values are translated).

Instead, keep values as list items.

Test Plan: Viewed Herald transcripts, saw no (meaningful) change in rendering output.

Maniphest Tasks: T13480

Differential Revision: https://secure.phabricator.com/D20949