Improve the XAR Import UI display to make it scalable with large number of documents

Use case: imagine importing a XAR containing several thousands of wiki pages.

Current problems:

• The import UI loads the full XAR in memory to let the user select which pages to import
• The import UI lists all the pages imported successfully (and in error) one below the other once the import has been done.
• The import is done synchronously and this brings several problems:
  • It may time out (browser time, proxy time out, etc)
  • The user cannot know the status on the progress

We need to find more scalable solutions for the UI display.