Pass additional parameters to newuser_notify_siteadmin filter

[boonebgorges]

The newuser_notify_siteadmin filter allows modification of the email that gets sent at new user activation on Multisite. However, the filter is not very useful, as it does not pass along any information about the user being activated. The attached patch passes the $user_id as well as the $user object.

Might as well just pass $user. Looks good.

In [20101]:

Pass WP_User object to the newuser_notify_siteadmin filter. props boonebgorges. fixes #20162.