Blueprint permalinks

Currently blueprint urls are based on blueprint names. However names can be changed by editing the Name entry in +edit. In this way there are no stable urls and when somebody changes the name of a specification all the external pages that link there (such as a wiki page) need to be modified.

This can be prevented by using both urls based on names and based on permanent IDs. So, for example, both these two links would work:
  https://blueprints.launchpad.net/project/+spec/my-cool-specification (changeable)
  https://blueprints.launchpad.net/project/+spec/123456 (always the same)