[nginx/templates] Add WordPress Multisite subdir support

[hudokkow]

This collides with #3739

I'll rebase and push after #3739 goes in.

The code in question is

	# Rewrite requests to /wp-.* on subdirectory installs.
	if (!-e $request_filename) {
		rewrite /wp-admin$ $scheme://$host$uri/ permanent;
		rewrite ^/[_0-9a-zA-Z-]+(/wp-.*) $1 last;
		rewrite ^/[_0-9a-zA-Z-]+(/.*\.php)$ $1 last;
	}