add slugs to urls

This adds optional slugs to the urls for canon and wiki pages.  The format would look like "/wiki/18/cabbage-enhancement-proposals/" and the slug would be optional, similar to how Stack Overflow does it.

Could also add "strong slugs" that identify an instance without needing the id, but not sure if that's necessary.  That would add an extra form and model field to specify a custom slug that must be unique for the model.