Add functions for *uploading* media (but not sending)

- This is a large part of the work toward being able to send images/video/etc.
- We can now encrypt stuff according to whatsapp's requirements, ask for an
  upload slot, and do the uploading (but the actual uploading is outsourced to
  the library consumer right now)
- Still missing, however, is the ability to stuff the necessary bits of info
  into a WhatsApp protobuf message and actually send it.