Import page: callback endpoint for upload

[ebruchez]

This calls for adding an optional callback endpoint once a user has uploaded a file on the Import page.

• add new configuration property
• implement callback
  • call endpoint
  • pass URL parameters

Useful information should be passed to the call:

• body of the uploaded file
• mediatype
• filename
• document-id if passed to the page
• app/form/version
• current language
• possibly: username?
• optionally: forward some headers, including Cookie

+1 from customer