Make New Site & New User Network form pluggable.

[ptahdunbar]

Problem: Currently the only way to add additional fields to the new site & new user network form is by dynamically injecting them using javascript.

Solution: They should either have action hooks or use the settings API. I'll provide patches for both methods.

In addition, I've added semantic ids to the current form fields so they can be selectively hidden.

[ptahdunbar]

Axing the settings api approach as that'll involve moving all the form processing to a new network/post.php file for just these two forms.

[ptahdunbar]

Alternatively, we could just have this within the forms:

do_settings_fields( 'wpms_new_site', 'default' );

Then:

add_settings_field( 'twitter-id', 'Twitter username', 'wpms_twitter_callback', 'wpmu_new_site', 'default' );

Which works, but that's only using half of the settings API as it won't make use of register_setting() (you'd have to handle form processing on your own).

[ptahdunbar]

This patch uses the hooks approach

[jeremyfelt]

Having a hook on both these pages would be great. In the standard user-new.php, we fire user_new_form after the closing table tag. It may be worth having some consistency, though per [25631] via #18709, this is a different form. Firing that outside the table would allow for more flexible output by plugins.

We should consider hitting this in 3.9 if possible.

[jeremyfelt]

Going to push this to another release so that we can make sure we do it the right way. Ideally, this would be refactored quite a bit to make any extension easier. As it is, any new action added to the form would need a matching action to handle it on save to avoid even more hacky implementations.

[jeremyfelt]

I think we can use the same approach for users as we did for sites in #34739. 15389.diff​ does this.

[jeremyfelt]

In 36556:

Multisite: Add a hook to the end of the network's Add New User form.

The network_user_new_form action can be used to extend the new user form.

Props ptahdunbar.
Fixes #15389.