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/Forum/ForumServiceProvider.php

@@ -13,7 +13,7 @@ namespace Flarum\Forum;
 use Flarum\Core\Users\Guest;
 use Flarum\Events\RegisterForumRoutes;
 use Flarum\Http\RouteCollection;
-use Flarum\Http\UrlGenerator;
+use Flarum\Forum\UrlGenerator;
 use Flarum\Support\ServiceProvider;
 use Psr\Http\Message\ServerRequestInterface;
 
@@ -31,7 +31,7 @@ class ForumServiceProvider extends ServiceProvider
         });
 
         $this->app->singleton(
-            'Flarum\Http\UrlGeneratorInterface',
+            UrlGenerator::class,
             function () {
                 return new UrlGenerator($this->app->make('flarum.forum.routes'));
             }
@@ -62,67 +62,67 @@ class ForumServiceProvider extends ServiceProvider
 
         $routes->get(
             '/all',
-            'flarum.forum.index',
+            'index',
             $defaultAction = $this->action('Flarum\Forum\Actions\IndexAction')
         );
 
         $routes->get(
             '/d/{id:\d+(?:-[^/]*)?}[/{near:[^/]*}]',
-            'flarum.forum.discussion',
+            'discussion',
             $this->action('Flarum\Forum\Actions\DiscussionAction')
         );
 
         $routes->get(
             '/u/{username}[/{filter:[^/]*}]',
-            'flarum.forum.user',
+            'user',
             $this->action('Flarum\Forum\Actions\ClientAction')
         );
 
         $routes->get(
             '/settings',
-            'flarum.forum.settings',
+            'settings',
             $this->action('Flarum\Forum\Actions\ClientAction')
         );
 
         $routes->get(
             '/notifications',
-            'flarum.forum.notifications',
+            'notifications',
             $this->action('Flarum\Forum\Actions\ClientAction')
         );
 
         $routes->get(
             '/logout',
-            'flarum.forum.logout',
+            'logout',
             $this->action('Flarum\Forum\Actions\LogoutAction')
         );
 
         $routes->post(
             '/login',
-            'flarum.forum.login',
+            'login',
             $this->action('Flarum\Forum\Actions\LoginAction')
         );
 
         $routes->post(
             '/register',
-            'flarum.forum.register',
+            'register',
             $this->action('Flarum\Forum\Actions\RegisterAction')
         );
 
         $routes->get(
             '/confirm/{token}',
-            'flarum.forum.confirmEmail',
+            'confirmEmail',
             $this->action('Flarum\Forum\Actions\ConfirmEmailAction')
         );
 
         $routes->get(
             '/reset/{token}',
-            'flarum.forum.resetPassword',
+            'resetPassword',
             $this->action('Flarum\Forum\Actions\ResetPasswordAction')
         );
 
         $routes->post(
             '/reset',
-            'flarum.forum.savePassword',
+            'savePassword',
             $this->action('Flarum\Forum\Actions\SavePasswordAction')
         );
 
@@ -137,7 +137,7 @@ class ForumServiceProvider extends ServiceProvider
 
         $routes->get(
             '/',
-            'flarum.forum.default',
+            'default',
             $defaultAction
         );
     }