Index Herald fields, not just actions, when identifying objects related to a particular Herald rule

Summary:
Fixes T13408. Currently, when a package (or other object) appears in a field (rather than an action), it is not indexed.

Instead: index fields too, not just actions.

Test Plan:

• Wrote a rule like "[ Affected packages include ] ...".
• Updated the search index.
• Saw rule appear on "Affected By Herald Rules" on the package detail page.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13408

Differential Revision: https://secure.phabricator.com/D20795