Massively simplify API binding logic

Changes the API internals to use normal Laravel binding which automatically supports nested-models and can determine their relationships. This removes a lot of confusingly complex internal logic and replaces it with standard Laravel code.

This also removes a deprecated "getModel" method and fully replaces it with a "parameter" method that does stricter type-checking.

app/Http/Middleware/Api/Client/SubstituteClientBindings.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Pterodactyl\Http\Middleware\Api\Client;
+
+use Closure;
+use Pterodactyl\Models\Server;
+use Illuminate\Routing\Middleware\SubstituteBindings;
+
+class SubstituteClientBindings extends SubstituteBindings
+{
+    /**
+     * @param \Illuminate\Http\Request $request
+     *
+     * @return mixed
+     */
+    public function handle($request, Closure $next)
+    {
+        // Override default behavior of the model binding to use a specific table
+        // column rather than the default 'id'.
+        $this->router->bind('server', function ($value) {
+            return Server::query()->where(strlen($value) === 8 ? 'uuidShort' : 'uuid', $value)->firstOrFail();
+        });
+
+        $this->router->bind('user', function ($value, $route) {
+            /** @var \Pterodactyl\Models\Subuser $match */
+            $match = $route->parameter('server')
+                ->subusers()
+                ->whereRelation('user', 'uuid', '=', $value)
+                ->firstOrFail();
+
+            return $match->user;
+        });
+
+        return parent::handle($request, $next);
+    }
+}