Fix [Object object] placeholder text in topic reply field

Change-Id: I51e4c8586be8995f5fbb32970f23fe9eec596186
Placeholder should only contain text of exactly the topic being replied to

When replying to a higher-level post, it would include the contents
of all lower-level posts as well. It should only fetch text of the
first post, no children.

Change-Id: I978736bb2ae194389709a7ad0029d14400eae30a