Simplify interface of migration-related classes

Mostly, we only need a database connection, instead of one of
Laravel's "connection resolvers".

Again, this makes our life easier during installation, where
we already instantiate a database connection. We can now use
that to instantiate our own Migrator class, instead of using
the IoC container to build one.

src/Database/MigrationServiceProvider.php

@@ -22,7 +22,7 @@ class MigrationServiceProvider extends AbstractServiceProvider
     public function register()
     {
         $this->app->singleton('Flarum\Database\MigrationRepositoryInterface', function ($app) {
-            return new DatabaseMigrationRepository($app['db'], 'migrations');
+            return new DatabaseMigrationRepository($app['flarum.db'], 'migrations');
         });
 
         $this->app->bind(MigrationCreator::class, function (Application $app) {