"Select All" checkbox in MyDownloadsPage can be checked with no selectable items

[marcellamaki]

Observed behavior

Reported by @nucleogenesis during the beta0 bug bash

when all items in the table are disabled, the “Select all” checkbox is not disabled, so I can still manage to select everything using that checkbox — noting it, but if the rows weren’t disabled, then this is behaving as intended beside the fact that clicking “Remove Selected” does nothing

https://www.notion.so/learningequality/Cannot-remove-item-I-selected-to-download-on-My-Downloads-page-7e87f388f8a74df4a6263f0bfd6f884a?pvs=4

Errors and logs

Expected behavior

"Select all" should only be enabled if one or more items are selectable

User-facing consequences

Confusion, mis-selecting items

Steps to reproduce

Initiate a download and navigate to the "My Downloads" page

Context

beta0

[nucleogenesis]

Fair chance that the issue causing this is the same as #11029