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/Core/Users/Commands/RequestPasswordResetHandler.php

@@ -17,7 +17,7 @@ use Illuminate\Contracts\Mail\Mailer;
 use Illuminate\Mail\Message;
 use Illuminate\Database\Eloquent\ModelNotFoundException;
 use Flarum\Core;
-use Flarum\Http\UrlGeneratorInterface;
+use Flarum\Forum\UrlGenerator;
 
 class RequestPasswordResetHandler
 {
@@ -37,7 +37,7 @@ class RequestPasswordResetHandler
     protected $mailer;
 
     /**
-     * @var UrlGeneratorInterface
+     * @var UrlGenerator
      */
     protected $url;
 
@@ -45,9 +45,9 @@ class RequestPasswordResetHandler
      * @param UserRepository $users
      * @param SettingsRepository $settings
      * @param Mailer $mailer
-     * @param UrlGeneratorInterface $url
+     * @param UrlGenerator $url
      */
-    public function __construct(UserRepository $users, SettingsRepository $settings, Mailer $mailer, UrlGeneratorInterface $url)
+    public function __construct(UserRepository $users, SettingsRepository $settings, Mailer $mailer, UrlGenerator $url)
     {
         $this->users = $users;
         $this->settings = $settings;

src/Core/Users/User.php

@@ -328,7 +328,7 @@ class User extends Model
      */
     public function getAvatarUrlAttribute()
     {
-        $urlGenerator = app('Flarum\Http\UrlGeneratorInterface');
+        $urlGenerator = app('Flarum\Forum\UrlGenerator');
 
         return $this->avatar_path ? $urlGenerator->toAsset('avatars/'.$this->avatar_path) : null;
     }