bz19958: fixed conversion menu.

Made a new way of handling the conversion items.  Now the backend
sends a list of (identifier, title) tuples to the frontend.  I think
this is cleaner than the frontend accessing conversion_manager.
Made the item context menu use the same system as the conversion menu
Fixed some side issues of #19939

If the we see a relative Location: header, then don't consider that an
invalid redirect (although it technically is).  Instead, just log a
warning and continue.

Don't consider InvalidRedirect and TooManyRedirects a connection
error.