Sequential numbers aren't great because they change when an earlier
comment is archived. Parent comment/heading IDs should change less
often.
This also makes much more sense for disambiguating subsections,
e.g. a dozen identical ===Votes=== sections for a dozen proposals.
Bug: T264478
Change-Id: I466454984fd919ebef35f2b37ddb5d86dc842996
Previously, only comments could have IDs, because we only needed IDs
for replying. But we might also use them for notifications soon.
Bug: T264478
Change-Id: I1bcad02bf17ab54bc5028a959543c10f0430836b
