Phriction basics

Summary:
Basically a copy/paste of parts of D636, but with two changes:

• Fully separate the index table ("document") from the content table

("content"). I think this will be a cleaner solution in the long run.

• Build slugs into the document structure.

This doesn't do anything useful, it just normalizes slugs and lays some
groundwork.

Test Plan:

• Visited various /w/ pages and saw them normalize correctly.
• Verified the DAO works by inserting dummy rows.

Reviewed By: codeblock
Reviewers: hsb, codeblock, jungejason, aran, tuomaspelkonen
CC: aran, codeblock, epriestley
Differential Revision: 638