Introduce is_blog_admin()

[ryan]

To compliment is_network_admin(). Have is_admin() return true for any admin page, network or blog.

(In [15558]) is_blog_admin(). fixes #14763

Should all of that be "site" instead of "blog"?

In the code, we're quite consistent with "blog" meaning site, and "site" meaning network. Also, is_site_admin() already exists, it was MU's is_super_admin(). Using "site" here would be very confusing when working with the rest of the MU code that includes things like site_id and such.

Oh yeah, right! :-) Forgot about that part of the mix-up. I had remember the switch from "blog" to "site", but apparently in the wrong place. Sorry.

Maybe because I'm an MU kinda guy, does in_blog_admin() and in_network_admin() make more sense for these? Particularly because of the is_site_admin/is_super_admin type functions from mu/ms being user focused template-tag style functions?