IdPUpgradesViaRewrite

This is mainly a placeholder, because I was initially unsuccessful trying to use URL rewriting as a way of making a single IdP instance live at different paths.

In particular, I had difficulties with cookies because the servlet container would write them using the "real" context path and not the one I was trying to emulate. If I understood mod_rewrite better, I'm sure it would have worked, but somebody using this approach will need to document it.