GUI: fixed URLs for password reset application

- PwdReset URL can now be set in perun-web-gui.properties, then only single
  URL is used. If value not set in config file, then calculate relative URL from
  hostname and authz type.

Following properties may be optionally added to perun-web-gui.properties before
this commit can be merged:

getPasswordResetUrl = [URL to pwd-reset application]