In Zulip, message topics are case-insensitive but case-preserving.
The `get_context_for_message` function erroneously did a
case-sensitive search, and thus only messages whose topic matched
exactly were pulled in as context.
Make the missed-message pipeline aware that message topics are not
case-sensitive. This means that, when collapsing adjacent messages,
we merge messages with topic headers which are "different"; create a
separate explicit "grouping" to know which to collapse.
e3c976fed9