For multisite, register admin menu with the 'network_admin_menu' hook.

This allows the "Confirm User Registration" admin page to be viewed
under the "Network Admin > Users" menu.

Also, this commit moves the admin scripts to the admin_menu() method.
Switch out single-site option calls for network-site option calls.

Using the network-site option function calls should be backwards-
compatible with single-site and allows for the plugin to work on
multisite installs.
Alter 'Edit' user link to use network_admin_url().

More multisite compatibility!
Alter 'Delete' user link to be multisite-compatible.

Created a new class method - delete_user_link() - to do this.
Tweak how current users are set as authenticated on first activation.

(1) Introduces new class method - get_users()

This class method replaces the get_authed_users() and get_pending_users()
methods, is more flexible and is multisite-compatible.

(2) Introduces new class method - set_current_users_as_authenticated()

This class method grabs all non-authenticated users and sets the user
meta for these users in two database queries.

Before, user meta was set in a foreach loop, which could result in
numerous database queries for large member installs.
Tweak what happens when a user is authenticated.

* Remove get_users() call as the passed $user object is already a WP_User
  object.
* Use WP standards for conditional braces.
Do not use get_userdata() function call.

We already have a WP_User object so pass that to WP_User to prevent
querying the database.
Change various class method visibility to protected.

These class methods are only used internally and shouldn't be public.
Fix notice.
Do not allow 'Delete' bulk action on multisite.

This doesn't work properly.  Multisite administrators should use the
main "Users" dashboard to manage user deletion instead.
Merge branch 'multisite'