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Introduce comment "names" to identify comments across revisions/pages

Description

Introduce comment "names" to identify comments across revisions/pages

The existing comment IDs can't be used to find the same comment on
a different revision or page (when it's transcluded), because they
depend on the comment's parent and its position on the page.

Comment names depend only on the author and timestamp. The trade-off
is that they can't distinguish comments posted within the same minute,
or in the same edit, so we will still need the IDs sometimes.

Prefer using comment names when replying, if they're not ambiguous.
This fixes T273413 and T275821.

Heading names depend on the author and timestamp of the oldest comment.
This way we don't have to detect changes to the heading text, but we
can't distinguish headings without any comments.

Bug: T274685
Bug: T273413
Bug: T275821
Change-Id: Id85c50ba38d1e532cec106708c077b908a3fcd49