autoconfig: set browser.newtabpage.enabled (MR 19)

Now that there's a nice about:home page, show it in new tabs too instead
of the weird looking blank page. Set this in autoconfig.js instead of
policies.json, so it can be overridden by the user via settings page.

src/mobile-config-autoconfig.js

@@ -61,3 +61,7 @@ defaultPref('browser.urlbar.suggest.topsites', false);
 // this option, at least there is no big "Search with Amazon" message in the
 // urlbar results as soon as typing the letter "a".
 defaultPref('browser.urlbar.suggest.engines', false);
+
+// Show about:home in new tabs, so it's not just a weird looking completely
+// empty page.
+defaultPref('browser.newtabpage.enabled', true);

src/policies.json

@@ -4,7 +4,6 @@
         "DisableFirefoxStudies": true,
         "DisableTelemetry": true,
         "DisablePocket": true,
-        "NewTabPage": false,
         "NoDefaultBookmarks": true,
         "OverrideFirstRunPage": "",
         "OverridePostUpdatePage": "",