Don't show Customizer link in network toolbars

[afragen]

r33130 adds a Customizer link to all toolbars. In multisite, this is also added to all Network screens too. I'm not sure this is the desired result.

Sorry for any issues with the patch as this is my first attempt. :-)

[helen]

@afragen There's a function for that :) is_network_admin(). Would also need to update the comment above it.

[afragen]

Thanks @helen. For some reason I was staring at Query Monitor's reporting is_network_admin() and is_main_network() ands seeing the same thing.

I will update patch.

[helen]

Going to turn it off for the admin completely per UI chat, so the commit is going to look a little different :)

[helen]

In 33147:

Toolbar: Don't show the customize link in the admin.

props afragen.
fixes #32945.

[ryan]

Aww, I was looking forward to finally having Customize available in the toolbar for site admin views, especially on phones.

[ryan]

Replying to ryan:

Aww, I was looking forward to finally having Customize available in the toolbar for site admin views, especially on phones.

I lost track of what "it" was in the chat about this.

​https://wordpress.slack.com/archives/design/p1436465948001446

"It" is the customize icon in the toolbar for admin views. Seems the decision was to omit Customize from all admin views while we figure out the future in #32678. Okay, cool. My interpretation of the mockups and discussion on #32678 is that we'll likely have Customize back in the site admin toolbar once we decide to remove customize links from the admin menu, eliminating all Appearance vs. Customize context mixing. Network admin would continue omitting Customize from the toolbar, of course.

[afragen]

Yeah, my first props!

[celloexpressions]

We'll need at least one release (realistically more than that) with Customize always available in the admin bar before we can remove it from the admin menu under Appearance. So that would be a good reason to keep it there now in addition to adding it on the front-end, or we could hold off for now and just push back that potential future change further. It seems pretty likely that we'll have the persistent customize link in the admin bar, so I'm leaning more towards keeping it in the (non-network) admin (well, now adding it back).