add missing allowedExtensions in MLContent

[mjauvin]

• fixes Text files in Content section OctoberCMS 1.1 pages-plugin#453
• fix test
• update migration files with incorrect syntax

[mjauvin]

@LukeTowers This has been handled in Plugin.php for a while now.

        /*
         * Handle translated page URLs
         */
        Page::extend(function($page) {
            if (!$page->propertyExists('translatable')) {
                $page->addDynamicProperty('translatable', []);
            }
            $page->translatable = array_merge($page->translatable, ['title', 'description', 'meta_title', 'meta_description']);
            $page->extendClassWith('RainLab\Translate\Behaviors\TranslatablePageUrl');
            $page->extendClassWith('RainLab\Translate\Behaviors\TranslatablePage');
        });

This was causing the test to fail because the Page class had already been extended with the TranslatablePage behavior.

[LukeTowers]

@mjauvin just as an FYI, I know it doesn't really matter since migration files should probably never be extended, but I would prefer we always use static:: over self:: unless there's a really good reason to use self::. It's a better practice to keep since that will provide the most flexibility for developers looking to extend the code.