I18n: get_locale calls involve much boilerplate code

When calling `get_locale` Jetpack code often checks for existence of the `GL_Locales` class and requires the file if it's not defined. This code is used in several places and can be abstracted to make everything more DRY:

```
if ( !class_exists( 'GP_Locales' ) ) {
if ( !defined( 'JETPACK__GLOTPRESS_LOCALES_PATH' ) || !file_exists( JETPACK__GLOTPRESS_LOCALES_PATH ) ) {
return false;
}
require JETPACK__GLOTPRESS_LOCALES_PATH;
}
```