Fix a couple of other missing getApplicationTransactionCommentObject() implementations

Summary:
See PHI165. See D18715. These objects (projects, blogs) also need implementations now.

(I thought about making this method abstract or doing try/catch to maybe make this more robust, but I think this should be the end of it, and those changes have mild complexity/compatibility/risk issues.)

Test Plan: Changed bin/search index to index only one document of each type, ran bin/search index --all --force, saw no more comment-related errors.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18729