R1:d40007aa32ae

Fix an issue where the Herald test console doesn't work with "Content source" rules

Summary:
Ref T13130. See PHI619. Currently, the Herald "Test Console" doesn't pass a "Content Source" to the adapter, so if any rules of the given type execute a "Content source" field rule, they'll fatal.

Provide a content source:

- If possible, use the content source from the most recent transaction.
- Otherwise, build a default "web" content source from the current request.

Test Plan:
- Wrote a "When [content…