In order for comments (AKA "page notes") to be discoverable with the
search logic implemented in #2413 (cc4ca35), they need to have at least
one target with a source field.

Previously a comment was defined as an annotation

- with a `uri`
- without any targets
- without any references (no targets but with references == a reply)

Now comments have a single target with a single field, `source`. This
finally puts us in a position where we can consider deprecating the
`uri` field.