This commit fixes a bug where a standalone annotation page only receives
realtime updates for replies on that page if the top-level annotation on
the page is also the root of its message thread.

The annotation `references` field contains the ids of all ancestors in
the message thread, from oldest to newest -- that is, the earlier in the
references field an id appears, the higher it is in the message tree.

In order to receive realtime updates for all messages in the part of the
thread displayed on a standalone annotation page, we need to ask for:

- all updates in which the id in the URL matches the id in the
  annotation (i.e. updates to the top-most displayed annotation, which
  may itself be a reply).

- all updates in which the id in the URL *is contained in* the
  `references` field of an annotation (i.e. updates for any annotation
  lower in the message thread)

Previously, this code requested all updates in which the id *was the
first entry* in the references field, which is only ever true for the
root of a message thread, meaning that it only ever worked on standalone
annotation pages for top-level annotations, and not on pages for
replies.