Update WordPress tracking commit
Update .gitignore to ignore extra plugins
Implement a replacement for wp-admin/network/site-new.php

Our rules around adding new sites to a network will be a little looser than those currently offered by WordPress core. By taking over the site-new.php requests, we can do what we want for the time being and attempt to develop a model that is useful to more than us.
Remove new user email language until we're more sure on handling

We may have a completely different process for finding and creating admin users when new sites are created.
Use DOCUMENT_URI rather than REQUEST_URI

Allows us to detect the same URL request without worrying about query parameters
Modify noncing and action to support `add-network-site` action
Remove unneeded hook into wp_admin_referer

We're going to handle this with a new action name instead so that we can avoid mucking things up.
Check for POSTed site data before continuing
Implement our version of the add-blog process from site-new.php

This allows for both a site domain and path to be submitted for a network.
Update tracking commit on WordPress
Either domain or path can be empty, just not both