Context: https://www.gerritcodereview.com/design-docs/attention-set.html
This change adds code for NoteDb to store and read attention sets.
We considered storing this data in the file that also holds the
comments (adding a dedicated file would require significant
refactoring). This would better accomodate the structured attention set
data, but this file is attached to a specific patch set whereas the
attention set relates to the change as a whole. We agreed that the
commit message is the better storage location, even when this is the
first footer line that will have JSON in it and be potentially long.
Change-Id: I9e48cf6c1bb6d6d8f28f9ca251cac2c3529af4c8
361ea4b790