In Herald transcripts, render some field values in a more readable way

Summary:
Ref T13480. Currently, some Herald field types are rendered in an unfriendly way on transcripts. Particularly, PHID lists are rendered as raw PHIDs.

Improve this by delegating rendering to Value objects and letting "PHID List" value objects render more sensible handle lists. Also improve "bool" fields a bit and make more fields render an explicit "None" / empty value rather than just rendering nothing.

Test Plan: Viewed various transcripts, including transcripts covering boolean values, the "Always" condition, large blocks of text, and PHID lists.

Maniphest Tasks: T13480

Differential Revision: https://secure.phabricator.com/D20951