gets the url of the current application (not including the context root). this setting
is derived from the value of the url parameter in the web section of the application config file,
or the xip.io url, if the use_xip_io config setting is enabled in the web section of the config file.
TODO
We should provide a means for non-finserve containers to provide the url.
We should keep a list of "preferred urls" that don't require changing when doing redirects.
As it stands now, if you have multiple ports serving the app, as soon as a redirect fires, you'll
get switched from that port to the "main port", which you might not be able to access.