Role/Capability: Add `is_user_member_of_blog` filter.

This aims to make it easier to dynamically grant or revoke a user access to the site without giving them a role directly.

Follow-up to [https://mu.trac.wordpress.org/changeset/804 mu:804], [19016].

Props dd32, mukesh27.
Fixes #65096.

git-svn-id: https://develop.svn.wordpress.org/trunk@62413 602fd350-edb4-49c9-b593-d223f7449a82