Add get_main_site_url() function from Cloud module to Core

The function `Altis\Cloud\get_main_site_url()` should really be at a lower level as it has more general usefulness and doesn't quite make sense under the `Cloud` namespace specifically.

It should be moved to this module under the `Altis` namespace directly then the Cloud module version deprecated.

Locate Altis\Cloud\get_main_site_url() in the Cloud module and inspect the Core module's namespace and tests. Move the function under the Altis namespace, preserve its behavior, deprecate the Cloud module version, and run the relevant Core and Cloud tests if present.

Written by the indexing model from the issue text.