Frontend refactor (#1471)

Refactor Frontend + Asset code

- Use Laravel's Filesystem component for asset IO, meaning theoretically
  assets should be storable on S3 etc.

- More reliable checking for asset recompilation when debug mode is on,
  so you don't have to constantly delete the compiled assets to force
  a recompile. Should also fix issues with locale JS files being
  recompiled with the same name and cached.

- Remove JavaScript minification, because it will be done by Webpack
  (exception is for the TextFormatter JS).

- Add support for JS sourcemaps.

- Separate frontend view and assets completely. This is an important
  distinction because frontend assets are compiled independent of a
  request, whereas putting together a view depends on a request.

- Bind frontend view/asset factory instances to the container (in
  service providers) rather than subclassing. Asset and content
  populators can be added to these factories – these are simply objects
  that populate the asset compilers or the view with information.

- Add RouteHandlerFactory functions that make it easy to hook up a
  frontend controller with a frontend instance ± some content.

- Remove the need for "nojs"

- Fix cache:clear command

- Recompile assets when settings/enabled extensions change

src/Frontend/Compiler/LessCompiler.php

@@ -0,0 +1,96 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Frontend\Compiler;
+
+use Flarum\Frontend\Compiler\Source\FileSource;
+use Less_Parser;
+
+class LessCompiler extends RevisionCompiler
+{
+    /**
+     * @var string
+     */
+    protected $cacheDir;
+
+    /**
+     * @var array
+     */
+    protected $importDirs = [];
+
+    /**
+     * @return string
+     */
+    public function getCacheDir(): string
+    {
+        return $this->cacheDir;
+    }
+
+    /**
+     * @param string $cacheDir
+     */
+    public function setCacheDir(string $cacheDir)
+    {
+        $this->cacheDir = $cacheDir;
+    }
+
+    /**
+     * @return array
+     */
+    public function getImportDirs(): array
+    {
+        return $this->importDirs;
+    }
+
+    /**
+     * @param array $importDirs
+     */
+    public function setImportDirs(array $importDirs)
+    {
+        $this->importDirs = $importDirs;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function compile(array $sources): string
+    {
+        if (! count($sources)) {
+            return '';
+        }
+
+        ini_set('xdebug.max_nesting_level', 200);
+
+        $parser = new Less_Parser([
+            'compress' => true,
+            'cache_dir' => $this->cacheDir,
+            'import_dirs' => $this->importDirs
+        ]);
+
+        foreach ($sources as $source) {
+            if ($source instanceof FileSource) {
+                $parser->parseFile($source->getPath());
+            } else {
+                $parser->parse($source->getContent());
+            }
+        }
+
+        return $parser->getCss();
+    }
+
+    /**
+     * @return mixed
+     */
+    protected function getCacheDifferentiator()
+    {
+        return time();
+    }
+}