Register menu in network admin area only.

* Show menu page under the "Sites" menu in the network admin area
* Only users with the 'manage_sites' capability can view this page
Refactor install blog routine.

* Use wpmu_create_blog() instead of custom, antiquated code
* Use code from /network/site-new.php to validate blog address (fixes
  issues with subdirectory / subdomain installs
* Remove antiquated class methods
Use custom function to query a user's ID.

* Removes and replaces deprecated get_user_id_from_string() function.
Add missing global from commit 455be60.
Fix notice.
Remove unused globals.
Output proper site on admin page.

* Adds new helper static method - the_site() - to do this.
Disable annoying More Privacy Options plugin email to super admin.

For our bulk create plugin, we don't need MPO's email to be sent to
the super admin when the blog's privacy status is updated.
General code cleanup and validate form HTML.