Theme translations and blog localisation (#8437)

refs #5345, refs #3801

- Blog localisation
  - default is `en` (English)
  - you can change the language code in the admin panel, see https://github.com/TryGhost/Ghost-Admin/pull/703
  - blog behaviour changes depending on the language e.g. date helper format
  - theme translation get's loaded if available depending on the language setting
  - falls back to english if not available

- Theme translation
  - complete automatic translation of Ghost's frontend for site visitors (themes, etc.), to quickly deploy a site in a non-English language
  - added {{t}} and {{lang}} helper
  - no backend or admin panel translations (!)
  - easily readable translation keys - very simple translation
  - server restart required when adding new language files or changing existing files in the theme
  - no language code validation for now (will be added soon)
  - a full theme translation requires to translate Ghost core templates (e.g. subscriber form)
  - when activating a different theme, theme translations are auto re-loaded
  - when switching language of blog, theme translations are auto re-loaded

- Bump gscan to version 1.3.0 to support more known helpers

**Documentation can be found at https://themes.ghost.org/v1.20.0/docs/i18n.**

ghost/zip/test/fixtures/test-theme/locales/de.json

@@ -0,0 +1,3 @@
+{
+    "Top left Button": "Oben Links."
+}

ghost/zip/test/fixtures/test-theme/locales/en.json

@@ -0,0 +1,3 @@
+{
+    "Top left Button": "Left Button on Top"
+}

ghost/zip/test/zip-folder.test.js

@@ -40,7 +40,7 @@ describe('lib/fs: read csv', function () {
                         return done(err);
                     }
 
-                    files.length.should.eql(12);
+                    files.length.should.eql(13);
                     done();
                 });
             });