R1:9f5e6bee9037

Make the default behavior of getApplicationTransactionCommentObject() "return null" instead of "throw"

Summary:
Depends on D20115. See <https://discourse.phabricator-community.org/t/transaction-search-endpoint-does-not-work-on-differential-diffs/2369/>.

Currently, `getApplicationTransactionCommentObject()` throws by default. Subclasses must override it to `return null` to indicate that they don't support comments.

This is silly, and leads to a bunch of code that does a `try / catch` around it, and at…