The user interface only allows you to subscribe to level 2 headings.
But we would generate events for whatever heading was the closest,
If it was e.g. level 3, no one would receive that notification.
Now we generate events for the closest level 2 heading, or we don't
generate the event at all if there isn't one (if the only headings are
of level 3 and below, or level 1, or if the comment is added before
the first heading on the page).
Bug: T286736
Change-Id: Iae99853070e353ab81c9cc29ef1d53c877adfc66