Fix default value for showHighlights

The default was incorrectly set to null because the previous code was setting it to null, however that null value was used to then set it a second time to "always" Part of this refactor is making the code more clear so this logic is explicit. The defaultValue for showHighlight is in fact 'always'

src/annotator/config/index.js

@@ -159,7 +159,7 @@ const configDefinitions = {
   },
   showHighlights: {
     allowInBrowserExt: false,
-    defaultValue: null,
+    defaultValue: 'always',
     getValue: settings => settings.showHighlights,
   },
   notebookAppUrl: {

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

@@ -88,7 +88,7 @@ describe('annotator/config/index', function () {
             query: 'fakeValue',
             requestConfigFromFrame: null,
             services: null,
-            showHighlights: null,
+            showHighlights: 'always',
             sidebarAppUrl: 'fakeValue',
             subFrameIdentifier: 'fakeValue',
             theme: null,
@@ -172,7 +172,7 @@ describe('annotator/config/index', function () {
         query: null,
         requestConfigFromFrame: null,
         services: null,
-        showHighlights: null,
+        showHighlights: 'always',
         sidebarAppUrl: null,
         subFrameIdentifier: null,
         theme: null,