S3: upload folders

Feature #1682 provides the ability to select and upload multiple files with one `Upload Files...` action. But it doesn't support folders.

## Expected behavior

1. the `Upload Files...` action supports selecting one or more folders. Folders are handled as follows:
1. if the local folder paths `foo/bar/folder1/` and `zub/zam/folder2/` are selected, and the remote S3 prefix (folder) is `s3://bucket1/a/` ,
1. the Toolkit uploads the entire _recursive_ contents of local folder `foo/bar/folder1/` to remote path`s3://bucket1/a/folder1/`
1. the Toolkit uploads the entire _recursive_ contents of local folder `zub/zam/folder2/` to remote path`s3://bucket1/a/folder2/`
1. any other files that were individually selected are uploaded to `s3://bucket1/a/`

Start by locating the existing `Upload Files...` action described in Feature #1682 and trace how individually selected files are mapped to the remote S3 prefix. Extend the behavior to accept folders, preserve each selected folder's name, upload contents recursively, and keep individually selected files at the prefix root.

Written by the indexing model from the issue text.