Add multiple UrlGenerator classes for forum/api/admin

Spent quite a while looking into the best solution here and ended up going with three separate classes. Thanks to @Luceos for the PR that got this rolling (#518). My reasoning is:

- The task of routing and URL generation is independent for each section of the app. Take Flarum\Api\Users\IndexAction for example. I don't want to generate a URL to a Flarum route... I specifically want to generate a URL to an API route. So there should be a class with that specific responsibility.
- In fact, each URL generator is slightly different, because we need to add a certain prefix to the start (e.g. /api)
- This also allows us to get rid of the "flarum.api" prefix on each route's name.
- It's still DRY, because they all extend a base class.

At the same time, I could see no reason this needed to be "interfaced", so all of the classes are concrete.

Goes a long way to fixing #123 - still just a few places left remaining with hardcoded URLs.

src/Admin/AdminServiceProvider.php

@@ -11,7 +11,7 @@
 namespace Flarum\Admin;
 
 use Flarum\Http\RouteCollection;
-use Flarum\Http\UrlGenerator;
+use Flarum\Admin\UrlGenerator;
 use Illuminate\Support\ServiceProvider;
 use Psr\Http\Message\ServerRequestInterface;
 use Zend\Diactoros\Response\RedirectResponse;
@@ -26,7 +26,7 @@ class AdminServiceProvider extends ServiceProvider
     public function register()
     {
         $this->app->singleton(
-            'Flarum\Http\UrlGeneratorInterface',
+            UrlGenerator::class,
             function () {
                 return new UrlGenerator($this->app->make('flarum.admin.routes'));
             }
@@ -57,7 +57,7 @@ class AdminServiceProvider extends ServiceProvider
 
         $routes->get(
             '/',
-            'flarum.admin.index',
+            'index',
             $this->action('Flarum\Admin\Actions\ClientAction')
         );
     }