add asset.user to simplify assignment and retrieval of asset "speaker"

[hburgund]

For projects where there are a lot of assets added by admins from historical or other sort of archive or pre-existing source, it is very useful to be able to know who is talking. This data can be exposed to clients/users as needed in addition to being a useful management tool for the content.

• should be optional field
• use "firstname_lastname" as device_id for script uploading so we don't duplicate users
• need to add fields to POST users/ response:
  • id
  • firstname
  • lastname
  • email
  • device_id
  • client_type