Multisite: Introduce get_main_site_id().
This function can be used to easily get the main site ID of a given network via the optional $network_id parameter, which defaults to the current network. The existing is_main_site() now uses the new function internally and now accepts an optional $network_id parameter as well.
The main purpose of the new function at this point is to ensure that the WP_Network::$blog_id property is always set. Magic getters in the class have been adjusted to auto-fill the property when it is accessed and empty. Furthermore the function encapsulates logic that was previously part of ms_load_current_site_and_network() and has been replaced with a call to the function now.
Props spacedmonkey, jeremyfelt, johnjamesjacoby, flixos90.
Fixes #29684.