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 source][is][whatever]" rule for tasks.
• Ran test console against a task.
• Before: got a fatal trying to interact with the content source.
• After: transcript reports sensible content source.
  • Also commented out the "xaction" logic to test the fallback behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19411