Allow uploading directories, not just multiple files

[mturoci]

Is your feature request related to a problem? Please describe

Currently, q.site.upload allows for specifying a list of files to be uploaded in a batch. However, Wave server creates a separate uuid-named folder for each file. Sometimes it's needed to keep the dir structure though, especially when working with 3rd party JS libs that dynamically load scripts using relative paths.

Describe the solution you'd like

Upload the whole directory and keep the structure as is.

Describe alternatives you've considered

The obvious alternative is using public/private dirs, but these cannot be set during runtime, which is a deal breaker for me.

Note: Needed for making the Tour and Wave studio work in air-gapped/heavily constrained networks (external CDNs not allowed).