Use Webdav PUT for uploads in the web browser

- uses PUT method with jquery.fileupload for regular and public file
  lists
- for IE and browsers that don't support it, use POST with iframe
  transport
- implemented Sabre plugin to handle iframe transport and redirect the
  embedded PUT request to the proper handler
- added RFC5995 POST to file collection with "add-member" property to
  make it possible to auto-rename conflicting file names
- remove obsolete ajax/upload.php and obsolete ajax routes

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>