Add Support for Multisite and the PMPro Multisite Memberships Plugin

[ideadude]

Currently if you install the PMPro Multisite Memberships Plugin on some subsites on a network, it will manage approval on each site individually instead of checking the approval data at the main site.

We should add a filter in get user meta to get the user meta from the main site, which should fix this to use the main site's approval data when deciding if a user is approved or not.

This will also make it possible for subsites to approve new users (I think). We should test this and probably hide the approvals page on subsites.