In Web Client, use jQuery.ajax() to upload files

If we use FormData and jQuery.ajax() calls to upload a torrent,
we can stop bundling the jquery.form.js module. In addition, this
simplifies passing arguments in the headers s.t. rpc-server.c doesn't
have to look for the CSRF token as one of the multiparts.

This changes the upload POST behavior, so give it a new name (upload2).
The old function (upload) will be deprecated but kept until 2.90 so
that third-party web clients using the old POST semantics will have
time to update.

Bug #5290 <https://trac.transmissionbt.com/ticket/5290>

web/javascript/jquery/Makefile.am

@@ -4,7 +4,5 @@ dist_data_DATA = \
   jqueryui-1.8.16.min.js \
   jquery.min.js \
   jquery.contextmenu.min.js \
-  jquery.form.js \
-  jquery.form.min.js \
   jquery.transmenu.min.js \
   json2.min.js