Bulk import of users from csv

Create the ability to bulk import users from a csv.

Test on single site and multisite.
Test when some of the usernames and emails address are already present on the subsite or on a different subsite.

No files, entry points, or tests are identified in the issue. Start by locating the existing CSV import flow in this PHP plugin and establish how users and multisite membership are represented. Done means bulk user import works on single-site and multisite installations, including the stated username and email conflicts across subsites.

Written by the indexing model from the issue text.