Split up the installer logic

This is probably the most complicated way I could find to fix #1587.

Jokes aside, this was done with a few goals in mind:
- Reduce coupling between the installer and the rest of Flarum's
  "Application", which we are building during installation.
- Move the installer logic to several smaller classes, which can then
  be used by the web frontend and the console task, instead of the
  former hacking its way into the latter to be "DRY".
- Separate installer infrastructure (the "pipeline", with the ability
  to revert steps upon failure) from the actual steps being taken.

The problem was conceptual, and would certainly re-occur in a similar
fashion if we wouldn't tackle it at its roots.

It is fixed now, because we no longer use the ExtensionManager for
enabling extensions, but instead duplicate some of its logic. That is
fine because we don't want to do everything it does, e.g. omit
extenders' lifecycle hooks (which depend on the Application instance
being complete).

> for each desired change, make the change easy (warning: this may be
> hard), then make the easy change

- Kent Beck, https://twitter.com/kentbeck/status/250733358307500032

Fixes #1587.

src/Extension/Extension.php

@@ -11,12 +11,18 @@
 
 namespace Flarum\Extension;
 
+use Flarum\Database\Migrator;
 use Flarum\Extend\Compat;
 use Flarum\Extend\LifecycleInterface;
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Contracts\Support\Arrayable;
 use Illuminate\Support\Arr;
 use Illuminate\Support\Str;
+use League\Flysystem\Adapter\Local;
+use League\Flysystem\Filesystem;
+use League\Flysystem\FilesystemInterface;
+use League\Flysystem\MountManager;
+use League\Flysystem\Plugin\ListFiles;
 
 /**
  * @property string $name
@@ -307,6 +313,25 @@ class Extension implements Arrayable
         return realpath($this->path.'/assets/') !== false;
     }
 
+    public function copyAssetsTo(FilesystemInterface $target)
+    {
+        if (! $this->hasAssets()) {
+            return;
+        }
+
+        $mount = new MountManager([
+            'source' => $source = new Filesystem(new Local($this->getPath().'/assets')),
+            'target' => $target,
+        ]);
+
+        $source->addPlugin(new ListFiles);
+        $assetFiles = $source->listFiles('/', true);
+
+        foreach ($assetFiles as $file) {
+            $mount->copy("source://$file[path]", "target://extensions/$this->id/$file[path]");
+        }
+    }
+
     /**
      * Tests whether the extension has migrations.
      *
@@ -317,6 +342,19 @@ class Extension implements Arrayable
         return realpath($this->path.'/migrations/') !== false;
     }
 
+    public function migrate(Migrator $migrator, $direction = 'up')
+    {
+        if (! $this->hasMigrations()) {
+            return;
+        }
+
+        if ($direction == 'up') {
+            return $migrator->run($this->getPath().'/migrations', $this);
+        } else {
+            return $migrator->reset($this->getPath().'/migrations', $this);
+        }
+    }
+
     /**
      * Generates an array result for the object.
      *

src/Extension/ExtensionManager.php

@@ -222,26 +222,16 @@ class ExtensionManager
      * Runs the database migrations for the extension.
      *
      * @param Extension $extension
-     * @param bool|true $up
+     * @param string $direction
      * @return void
      */
-    public function migrate(Extension $extension, $up = true)
+    public function migrate(Extension $extension, $direction = 'up')
     {
-        if (! $extension->hasMigrations()) {
-            return;
-        }
-
-        $migrationDir = $extension->getPath().'/migrations';
-
         $this->app->bind('Illuminate\Database\Schema\Builder', function ($container) {
             return $container->make('Illuminate\Database\ConnectionInterface')->getSchemaBuilder();
         });
 
-        if ($up) {
-            $this->migrator->run($migrationDir, $extension);
-        } else {
-            $this->migrator->reset($migrationDir, $extension);
-        }
+        $extension->migrate($this->migrator, $direction);
     }
 
     /**
@@ -252,7 +242,7 @@ class ExtensionManager
      */
     public function migrateDown(Extension $extension)
     {
-        return $this->migrate($extension, false);
+        return $this->migrate($extension, 'down');
     }
 
     /**