Disable "Add to library" button on already reusable section

Problem/Motivation

Currently, it is possible to add an already reusable section to the section library again and mark it as reusable once more. This creates the potential for an infinite loop of reusable sections nested within each other, all pointing to the same original section. However, only the original section remains editable in this scenario (which you then have to find).

Proposed resolution

Best case: Remove/Disable the "Add to library" button in the section actions on a section that is already reusable.
Worst case: Remove/Disable the "Reusable" checkbox in the "Add section to library" form on a section that is already reusable.