*: allows for BASE_PATH configuration

This should unblock users who'd like to run the UI under a subpath on an existing vhost of their reverse proxy.
This is far from behind the most pretty/smart way to do it, but it works for all intent and purposes.
Fixes https://github.com/ngoduykhanh/wireguard-ui/issues/37

templates/global_settings.html

@@ -156,7 +156,7 @@ Global Settings
             $.ajax({
                 cache: false,
                 method: 'POST',
-                url: '/global-settings',
+                url: '{{.basePath}}/global-settings',
                 dataType: 'json',
                 contentType: "application/json",
                 data: JSON.stringify(data),