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 after a rename.

Test Plan:

• Applied migration.
• Created new posts.
• Edited existing posts.
• Visited various posts.
• Created a post with a bunch of "/" in the title, things still worked fine.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9995

Differential Revision: https://secure.phabricator.com/D14792