Theme deletion when running multisite

[nacin]

The delete link should be hidden from the themes list table when running multisite.

But then there's no way to delete a theme.

I think there should be a delete link and action added into the multisite list table. Probably just with a confirmation warning for now that other sites may be using that theme still, even if it is disabled in ms-themes.

I believe a deleted theme will kick them back to WP_DEFAULT_THEME.

If you delete it at the site admin level or manually, it does seem to fall back to WP_DEFAULT_THEME, so I imagine the same thing would happen if you deleted @ the net admin level.

Seems to make sense that the network admin would be where this would happen -- and eventually it should check network-wide for active themes.

Also seeing some strangeness with plugins at the Network Admin level, but that'll have to find a home in another ticket...

(In [16759]) Don't reference nonexistent tabs. see #15707.

15707.patch adds a delete link to ms themes with a confirmation warning. The link will be displayed if the theme isn't network enabled.

The big problem, there is no kick back to WP_DEFAULT_THEME. On site admin we have the function validate_current_theme() which checks the current theme.

How to fix this:

1. Remove the Delete link from the themes list table if running multisite.

2. Add the Delete link to the MS themes list table (patch here), but prevent deletion of the main site's active theme. (Hide the link and add a check in the post handler.)

Seems good. Following up with some esc_url() on all of the actions links might be prudent.

(In [17101]) Add delete support to network themes. Props PeteMall. fixes #15707