Give comments/page notes a single target with source field

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.

h/static/scripts/annotator/guest.coffee

@@ -296,6 +296,22 @@ module.exports = class Guest extends Annotator
   createHighlight: ->
     return this.createAnnotation({$highlight: true})
 
+  # Create a blank comment (AKA "page note")
+  createComment: () ->
+    annotation = {}
+    self = this
+
+    prepare = (info) ->
+      annotation.document = info.metadata
+      annotation.uri = info.uri
+      annotation.target = [{source: info.uri}]
+
+    this.getDocumentInfo()
+      .then(prepare)
+      .then(-> self.publish('beforeAnnotationCreated', [annotation]))
+
+    annotation
+
   showAnnotations: (annotations) ->
     tags = (a.$$tag for a in annotations)
     @crossframe?.call('showAnnotations', tags)

h/static/scripts/annotator/plugin/toolbar.coffee

@@ -57,7 +57,7 @@ module.exports = class Toolbar extends Annotator.Plugin
         "click": (event) =>
           event.preventDefault()
           event.stopPropagation()
-          @annotator.createAnnotation()
+          @annotator.createComment()
           @annotator.show()
     ]
     @buttons = $(makeButton(item) for item in items)

h/static/scripts/annotator/test/guest-test.coffee

@@ -240,6 +240,46 @@ describe 'Guest', ->
       annotation = guest.createAnnotation(annotation)
       assert.equal(annotation.foo, 'bar')
 
+    it 'triggers a beforeAnnotationCreated event', (done) ->
+      guest = createGuest()
+      guest.subscribe('beforeAnnotationCreated', -> done())
+
+      guest.createAnnotation()
+
+  describe 'createComment()', ->
+    it 'adds metadata to the annotation object', ->
+      guest = createGuest()
+      sinon.stub(guest, 'getDocumentInfo').returns(Promise.resolve({
+        metadata: {title: 'hello'}
+        uri: 'http://example.com/'
+      }))
+
+      annotation = guest.createComment()
+
+      timeoutPromise()
+      .then ->
+        assert.equal(annotation.uri, 'http://example.com/')
+        assert.deepEqual(annotation.document, {title: 'hello'})
+
+    it 'adds a single target with a source property', ->
+      guest = createGuest()
+      sinon.stub(guest, 'getDocumentInfo').returns(Promise.resolve({
+        metadata: {title: 'hello'}
+        uri: 'http://example.com/'
+      }))
+
+      annotation = guest.createComment()
+
+      timeoutPromise()
+      .then ->
+        assert.deepEqual(annotation.target, [{source: 'http://example.com/'}])
+
+    it 'triggers a beforeAnnotationCreated event', (done) ->
+      guest = createGuest()
+      guest.subscribe('beforeAnnotationCreated', -> done())
+
+      guest.createComment()
+
   describe 'anchor()', ->
     el = null
     range = null