Move settings repositories to own namespace

Also add a third method to the contract. This will help with building a caching decorator.

src/Core/Settings/DatabaseSettingsRepository.php

@@ -1,10 +1,10 @@
 <?php
 
-namespace Flarum\Core;
+namespace Flarum\Core\Settings;
 
 use Illuminate\Database\ConnectionInterface;
 
-class DatabaseSettingsRepository implements SettingsRepositoryInterface
+class DatabaseSettingsRepository implements SettingsRepository
 {
     protected $database;
 
@@ -13,6 +13,11 @@ class DatabaseSettingsRepository implements SettingsRepositoryInterface
         $this->database = $connection;
     }
 
+    public function all()
+    {
+        return $this->database->table('config')->lists('value', 'key');
+    }
+
     public function get($key, $default = null)
     {
         if (is_null($value = $this->database->table('config')->where('key', $key)->pluck('value'))) {

src/Core/Settings/SettingsRepository.php

@@ -1,9 +1,11 @@
 <?php
 
-namespace Flarum\Core;
+namespace Flarum\Core\Settings;
 
-interface SettingsRepositoryInterface
+interface SettingsRepository
 {
+    public function all();
+
     public function get($key, $default = null);
 
     public function set($key, $value);