Read apiUrl from services.

We need to be able to support the client to work with different annotation
services, other than just `https://hypothes.is/api`.
If the settings from the host doesn't specify a service, the client should
fall back to using the default apiUrl which would be `https://hypothes.is/api`.

https://github.com/hypothesis/product-backlog/issues/278

src/sidebar/app.js

 'use strict';
 
 var addAnalytics = require('./ga');
+var getApiUrl = require('./get-api-url');
 var serviceConfig = require('./service-config');
 require('../shared/polyfills');
 
@@ -19,6 +20,8 @@ if (settings.raven) {
 var hostPageConfig = require('./host-config');
 Object.assign(settings, hostPageConfig(window));
 
+settings.apiUrl = getApiUrl(settings);
+
 // Disable Angular features that are not compatible with CSP.
 //
 // See https://docs.angularjs.org/api/ng/directive/ngCsp

src/sidebar/get-api-url.js

+'use strict';
+
+var serviceConfig = require('./service-config');
+
+/**
+ * Function that returns apiUrl from the settings object.
+ *
+ * @param {object} settings - The settings object
+ * @returns {string} The apiUrl from the service or the default apiUrl from the settings
+ * @throws {Error} If the settings has a service but the service doesn't have an apiUrl
+ *
+ */
+function getApiUrl(settings) {
+  var service = serviceConfig(settings);
+
+  if (service) {
+    // If the host page contains a service setting then the client should default to
+    // using that apiUrl.
+    if (service.apiUrl) {
+      return service.apiUrl;
+    } else {
+      throw new Error('Service should contain an apiUrl value.');
+    }
+  }
+  return settings.apiUrl;
+}
+
+module.exports = getApiUrl;

src/sidebar/test/get-api-url-test.js

+'use strict';
+
+var getApiUrl = require('../get-api-url');
+
+describe('sidebar.getApiUrl', function () {
+  context('when there is a service object in settings', function () {
+    it('returns apiUrl from the service object', function () {
+      var settings = {
+        apiUrl: 'someApiUrl',
+        services: [{
+          apiUrl: 'someOtherApiUrl',
+        }],
+      };
+      assert.equal(getApiUrl(settings), settings.services[0].apiUrl);
+    });
+  });
+
+  context('when there is no service object in settings', function () {
+    it('returns apiUrl from the settings object', function () {
+      var settings = {
+        apiUrl: 'someApiUrl',
+      };
+      assert.equal(getApiUrl(settings), settings.apiUrl);
+    });
+  });
+
+  context('when there is a service object in settings but does not contain an apiUrl key', function () {
+    it('throws error', function () {
+      var settings = {
+        apiUrl: 'someApiUrl',
+        services: [{}],
+      };
+      assert.throws(
+        function() { getApiUrl(settings); },
+        Error,
+        'Service should contain an apiUrl value');
+    });
+  });
+});