Multisite: Add a filter to get_space_allowed() and increase its internal default value to the network default.

props jkudish for initial patch.
fixes #23650.


Built from https://develop.svn.wordpress.org/trunk@25092


git-svn-id: https://core.svn.wordpress.org/trunk@25076 1a063a9b-81f0-0310-95a4-ce76da25c4cd