Audit nonces

[ryan]

Audit for nonces, especially in network admin.

[ryan]

(In [17122]) nonce checks for ms themes. see #15969

[nacin]

(In [17126]) bulk-themes nonce for network/themes.php deletion. props PeteMall, see #15969.

[nacin]

The nonce added in r17126 duplicates another check in that branch after the confirmation screen. Not sure which should go.

[nacin]

(In [17127]) Remove this check in favor of the one added in [17126]. see #15969.

[nacin]

site-themes.php and site-users.php need nonce checks. I checked the rest of the network admin.

[PeteMall]

Nonce checks for site-themes.

[ryan]

(In [17134]) nonce checks for site-themes. Props PeteMall. see #15969

[PeteMall]

Nonce checks for site-users.

[westi]

Looks good here too. Committing.

[westi]

(In [17136]) Nonce checks for site-users. See #15969 props PeteMall.

[nacin]

If someone else can do a sweep of the network admin (I already have), that'd be great.

Please also sweep network-specific pieces in plugins.php and related pages.

[PeteMall]

Nonces in the network admin look good to me.