"Learning Activities" in `DetailsTabView`: Set default values on file upload

[sairina]

Observed behavior

According to the designs for the "Learning Activities" dropdown, there should be a default value set depending on the type of resource (audio, video, document).

PR #3370, the input for "Learning Activities" only allows the user to edit the "Learning Activities" input, but does not have a default set depending on kind.

Expected behavior

Setting the default should happen at file upload time instead.

User-facing consequences

Defaults are not currently set, and if this input is empty, an error shows up for the user.

Errors and logs

Additional information

I had originally used the getter for learning_activities in DetailsTabView to determine the default based on this.firstNode.kind. However, some issues to note that occurred were:

• the value was set in the UI, but not set in the backend, so validation always showed an error (that the field was empty)
• whenever the field was completely cleared, because of the default, we automatically saw the default pop pretty immediately

Because of this, we're tagging this as a follow-up issue that would finish the "Learning Activities" field (per specs) in the edit modal, and there is a connected issue for the backend to migrate data here.