R1:2d588715bc56

Always automatically generate Phame slugs

Summary:
Fixes T9995. I think letting users customize slugs is not a hugely compelling as a product feature, and this fixes the issue with slugs that have "/" characters in them and makes the move to EditEngine easier since I don't have to deal with the weird JS thing.

Instead, just generate slugs automatically. No more JS, no more separate field, things automatically update if you rename a blog, and now that URIs have IDs in them the old URI will still work…