Read annotations from the host page in settings.annotations()

Add code to the `settings.annotations()` method so that, instead of
reading the setting from the URL only, it reads the annotations setting
from the `js-hypothesis-config` scripts in the host page or, failing
that, tries to read it from the URL.

In `index.js`, after `settings.annotations()` is called, there are two
`Object.assign()` calls that overwrite values already in the `config`
objects with any returned from `js-hypothesis-config` scripts or the
`window.hypothesisConfig()` function in the host page. So a
`config.annotations` setting from `settings.annotations()` already gets
overwritten by a `annotations` setting in a `js-hypothesis-config`
script or `hypothesisConfig()` function.

This commit makes that overwriting explicit, encapsulates it in the
`annotations()` method, and unit tests it.

Note that the two `Object.assign()`s are still present in `index.js` so
the `config.annotations` setting returned by the `annotations()` method
still gets overwritten by them, but this now has no effect as the
`annotations()` method will already have set `config.annotations` to the
same value that it will be overwritten with.

In a future commit the `Object.assign()`s will be removed and there
won't be any overwriting.

Note that **this will change the behaviour of the code** once the
`Object.assign()`s have been removed: the `annotations()` method reads
the annotations setting from `js-hypothesis-config` scripts but does not
read it from `window.hypothesisConfig()` any longer. I don't believe
it's necessary to read `annotations` from `hypothesisConfig()`.

src/annotator/config/settings.js

@@ -46,13 +46,19 @@ function settingsFrom(window_) {
    * @return {string|null} - The extracted ID, or null.
    */
   function annotations() {
-    // Annotation IDs are url-safe-base64 identifiers
-    // See https://tools.ietf.org/html/rfc4648#page-7
-    var annotFragmentMatch = window_.location.href.match(/#annotations:([A-Za-z0-9_-]+)$/);
-    if (annotFragmentMatch) {
-      return annotFragmentMatch[1];
+
+    /** Return the annotations from the URL, or null. */
+    function annotationsFromURL() {
+      // Annotation IDs are url-safe-base64 identifiers
+      // See https://tools.ietf.org/html/rfc4648#page-7
+      var annotFragmentMatch = window_.location.href.match(/#annotations:([A-Za-z0-9_-]+)$/);
+      if (annotFragmentMatch) {
+        return annotFragmentMatch[1];
+      }
+      return null;
     }
-    return null;
+
+    return jsonConfigs.annotations || annotationsFromURL();
   }
 
   /**

src/annotator/config/test/settings-test.js

@@ -120,6 +120,24 @@ describe('annotator.config.settingsFrom', function() {
   }
 
   describe('#annotations', function() {
+    context('when the host page has a js-hypothesis-config with an annotations setting', function() {
+      beforeEach('add a js-hypothesis-config annotations setting', function() {
+        fakeSharedSettings.jsonConfigsFrom.returns({annotations: 'annotationsFromJSON'});
+      });
+
+      it('returns the annotations from the js-hypothesis-config script', function() {
+        assert.equal(settingsFrom(fakeWindow()).annotations, 'annotationsFromJSON');
+      });
+
+      context("when there's also an annotations in the URL fragment", function() {
+        specify('js-hypothesis-config annotations override URL ones', function() {
+          var window_ = fakeWindow('http://localhost:3000#annotations:annotationsFromURL');
+
+          assert.equal(settingsFrom(window_).annotations, 'annotationsFromJSON');
+        });
+      });
+    });
+
     [
       {
         describe: "when there's a valid #annotations:<ID> fragment",