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/Http/UrlGenerator.php

@@ -13,10 +13,11 @@ namespace Flarum\Http;
 
 use Flarum\Core;
 
-class UrlGenerator implements UrlGeneratorInterface
+class UrlGenerator
 {
     protected $routes;
 
+    protected $prefix;
 
     public function __construct(RouteCollection $routes)
     {
@@ -28,11 +29,11 @@ class UrlGenerator implements UrlGeneratorInterface
         $path = $this->routes->getPath($name, $parameters);
         $path = ltrim($path, '/');
 
-        return Core::url() . "/$path";
+        return Core::url($this->prefix) . "/$path";
     }
 
     public function toAsset($path)
     {
-        return Core::url() . "/assets/$path";
+        return Core::url($this->prefix) . "/assets/$path";
     }
 }