Pull request: client: use relative links for mobileconfig api

Updates #2905.

Squashed commit of the following:

commit af607433e07a43497930cdc1ef0012fa713e64da
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 1 19:33:35 2021 +0300

    client: use relative links for mobileconfig api

client/src/helpers/constants.js

@@ -646,6 +646,6 @@ export const isFirefox = navigator.userAgent.indexOf('Firefox') !== -1;
 export const COMMENT_LINE_DEFAULT_TOKEN = '#';
 
 export const MOBILE_CONFIG_LINKS = {
-    DOT: '/apple/dot.mobileconfig',
-    DOH: '/apple/doh.mobileconfig',
+    DOT: 'apple/dot.mobileconfig',
+    DOH: 'apple/doh.mobileconfig',
 };