DESIGN: massive rework of underlying concepts

This includes many difference changes, the most notable being:

* The addition of a "template descriptor" concept that removes the need
  for a lot of the previous hacks and adds quite a bit of extensability
  and flexability.
* The (tentative) removal of DNS-based redirection.
* The removal of URI templates (in favour of the new template
  descriptors).

Signed-off-by: Aleksa Sarai <asarai@suse.de>