Missing info for filesystem credential check on bulk deletion of themes in Multisite

[kobenland]

Context: Bulk deleting Themes in Network Admin.

When checking for filesystem credentials in delete_theme(), the redirect url is missing information about the files to delete, since it is passed through POST and not GET.

I experienced it in 3.3.2 and it still occurs in the latest nightly build.

[kobenland]

Not sure if passing of the entire $_POST variable is a good idea, but we need 'action', 'checked' and the nonce (at least)

[kobenland]

Alternativ with only the required parameters and a more static base url

[kobenland]

Added refactoring of redirect url

[dd32]

Patch looks good, and tests out OK for me on Multisite.

[dd32]

In [22084]:

Fix Bulk Theme Deletion in the Network Admin when using FTP. Props kobenland. Fixes #20671