Until now only a limited number of self-replies were inlined as an
anonymous, unordered ActivityPub collection. Notably the advertised
replies might be private posts.
However, providing all (non-private) replies allows for better thread
consistency across instances if the remote server cooperates.
The collection existing as a stndalone object has two advantages
for this. For one, if it was still anonymous, _all_ replies would need
to be inlined, which might be too bloated in pathological cases.
Secondly, it allows remote servers to update the thread by traversing
the reply collection independent of the original post. (If the remote
part knows about chronological ordering, it can in theory
even efficiently resume from where it previously stopped)
b41a13df56