Fix 'Transcript' links in Differential

Summary:
add filtering for MetaMTA transcripts, add Herald
transcripts, also fixed PhabricatorObjectHandleData to support commits.
Note that paging in the transcripts pages will be in a different diff.

Test Plan:
test the transcripts for both MetaMTA and Herald.

Reviewed By: epriestley
Reviewers: epriestley, tuomaspelkonen
CC: jungejason, epriestley
Differential Revision: 114