Allow ApplicationTransactionEditor to figure out whether TYPE_COMMENT is supported or not

Summary: See D17812, etc. We can figure this out by looking at the object carefully. We don't need to go delete all the old TYPE_COMMENT (it doesn't hurt anything) but can nuke it when we see it.

Test Plan:

• Made a comment in Slowvote (supports commenting).
• Viewed an Almanac device (does not support commenting).

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17822