Clean up usages / deprecate path helpers (#2155)

* Write source map without creating temp file

Less I/O, and one less place where we access the global path helpers.

* Drop useless app_path() helper

This was probably taken straight from Laravel. There is no equivalent
concept in Flarum, so this should be safe to remove.

* Deprecate global path helpers

Developers using these helpers can inject the `Paths` class instead.

* Stop storing paths as strings in container

* Avoid using path helpers from Application class

* Deprecate path helpers from Application class

* Avoid using public_path() in prerequisite check

a) The comparison was already outdated, as a different path was passed.
b) We're trying to get rid of these global helpers.

src/Formatter/FormatterServiceProvider.php

@@ -10,6 +10,7 @@
 namespace Flarum\Formatter;
 
 use Flarum\Foundation\AbstractServiceProvider;
+use Flarum\Foundation\Paths;
 use Illuminate\Cache\Repository;
 use Illuminate\Contracts\Container\Container;
 
@@ -24,7 +25,7 @@ class FormatterServiceProvider extends AbstractServiceProvider
             return new Formatter(
                 new Repository($container->make('cache.filestore')),
                 $container->make('events'),
-                $this->app['flarum']->storagePath().'/formatter'
+                $this->app[Paths::class]->storage.'/formatter'
             );
         });