Make Astronomy and Physics the default databases for new searches

This only applies if:

1. User is NOT logged in, and so has not updated their preferences
2. User is logged in but has not set their default database (or is still the default of none selected).

src/js/widgets/preferences/views/application.js

@@ -31,6 +31,10 @@ define([
           name: 'General',
           value: false,
         },
+        {
+          name: 'Earth Science',
+          value: false,
+        },
       ],
     },
     hideSidebars: {

src/js/widgets/search_bar/search_bar_widget.js

@@ -43,6 +43,13 @@ define([
   select2,
   oldMatcher
 ) {
+
+  /**
+   * The default databases to filter by if the user has not set any in their preferences
+   * @type {string[]}
+   */
+  const DEFAULT_DATABASES = ['Astronomy', 'Physics'];
+
   var SearchBarModel = Backbone.Model.extend({
     defaults: function() {
       return {
@@ -635,8 +642,8 @@ define([
         : this.defaultDatabases;
     },
 
-    applyDefaultFilters: function(apiQuery) {
-      var dbfilters = this.defaultDatabases || [];
+    applyDefaultFilters: function (apiQuery) {
+      const dbfilters = Array.isArray(this.defaultDatabases) && this.defaultDatabases.length > 0 ? this.defaultDatabases : DEFAULT_DATABASES;
       if (dbfilters.length > 0) {
         var fqString = '{!type=aqp v=$fq_database}';