Deleting a theme from a single site's Themes screen should not be allowed

[johnbillion]

With Multisite enabled, it's not possible to delete a plugin from the Plugins screen in individual sites' dashboards. The Delete link is removed.

The same should be true for themes. Currently, a Delete button is shown when a super admin views the Themes screen on an individual site.

[williampatton]

To me this seems like a good quality of life change and also a good idea to make behaviour between theme and plugin handling of this the same.

[Mista-Flo]

I have added a patch, it's quite simple, just check if we are in multisite or not to display the Delete button.

Also, I do not know if the screenshot is really needed since it would just be two screenshots, one with the button, the other without it.

[johnbillion]

41441.diff​ is a slightly different approach, it moves the ! is_multisite() logic to the point where the actions are constructed, therefore keeping the theme deletion logic in one place.

[johnbillion]

In 49011:

Themes: Remove the ability to delete themes from the single site Appearance screen on Multisite installations.

This brings theme deletion inline with plugin deletion and theme and plugin editing, which can only be performed from within Network Admin.

Props Mista-Flo, williampatton, johnbillion

Fixes #41441