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/SettingsRepository.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Flarum\Core\Settings;
+
+interface SettingsRepository
+{
+    public function all();
+
+    public function get($key, $default = null);
+
+    public function set($key, $value);
+}