Support export of user activity logs

Summary:
Depends on D18966. Ref T13049. Adds export support to user activity logs.

These don't have PHIDs. We could add them, but just make the "phid" column test if the objects have PHIDs or not for now.

Test Plan:

• Exported user activity logs, got sensible output (with no PHIDs).
• Exported some users to make sure I didn't break PHIDs, got an export with PHIDs.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18967