Add placeholder loading animation to expand button

[taylor-steve]

Increases the scope of the selector to target all turbo frames, not only the lazy loading turbo frames, in the collection context sidebar. This adds the loading animation above the "Expand" button once clicked:

The change from after to before does not change the existing behavior because all the loading="lazy" turbo frame containers in app/components/arclight/document_components_hierarchy_component.html.erb are empty.

I believe the docs suggest we can use busy in this way:

busy is a boolean attribute toggled to be present when a <turbo-frame>-initiated request starts, and toggled false when the request ends

This is something we are thinking of adding to our app. I thought I'd check if there was interest here first.

[marlo-longley]

Tested the branch and confirmed that this works to show the animation both for the original case of loading a long list of files after hitting the plus symbol:

and also for the Expand button case described above by @taylor-steve.