Implement Hooks and Filters

[swissspidy]

The original user-new.php script has lots of important filters and hooks. We need feature parity here to not break other plugins.

Hooks (both run twice for multisite and normal sites):

• user_new_form_tag — Fires inside the adduser form tag.
• user_new_form — Fires at the end of the new user form.

Applied Filters:

• show_password_fields — not needed in our case

Multisite only:

• pre_user_login — Filter the username before it is added to the site.
• autocomplete_users_for_site_admins — Filter whether to enable user auto-complete for non-super admins in Multisite.

Added Filters (esp. for Multisite):

• wpmu_signup_user_notification_email — Changes the notification email text
• wpmu_signup_user_notification — Disable confirmation mail
• wpmu_welcome_user_notification — Disable welcome mail

For Multisite support, see #4.