Automatically add main url to host-map.php

If subsites is used in conjunction with [staticpublishqueue](https://github.com/silverstripe-labs/silverstripe-staticpublishqueue), *host-map.php* currently has to be manually edited to include the main website's base URL every time a `Subsite` record is saved. Using these modules together already requires `FilesystemPublisher::$static_base_url` to be set, so we may as well make use of this setting if it is available to save this manual editing.