Add filter hook to 'is_super_admin()' function

[benward]

This patch adds a new filter, named 'is_super_admin' to the end of the capabilities.php:is_super_admin() function, allowing plugins to override the result of this capabilities check.

Use cases:

• Applying/restricting Super Admin privilege based on an external user authentication/permissions system.

• Restricting Super Admin privilege based on a user's network location.

[benward]

Adds 'is_super_user' filter to 'is_super_user()' function.

[mordauk]

I'm not confident we should be giving plugins the power to do that so easily. Right now if a plugin wanted to give a user super admin rights, it could use the ​grant_super_admin() function.

I'd rather plugins be required to promote a user to a super admin instead of allowing them to simply filter the is_super_admin check.

[nacin]

Yeah, I agree. We should also work toward making more is_super_admin() calls capability checks instead.