Basic support for DbMessageSource

[brandonkelly]

Description

Adds a setup/message-tables command, which sets the database up to use DbMessageSource for site translations.

Once run, the site translation category can be reconfigured via config/app.php like so:

use yii\helpers\UnsetArrayValue;
use yii\i18n\DbMessageSource;

return [
    'components' => [
        'i18n' => [
            'translations' => [
                'site' => [
                    'class' => DbMessageSource::class,
                    // DbMessageSource doesn't have a $basePath property,
                    // so remove it from the default config
                    'basePath' => new UnsetArrayValue(),
                ],
            ],
        ],
    ],
];

New site source messages and translations can then be added to the new source_message and message tables, and will be returned via Craft::t('site', 'source message') and the |t Twig filter.