Shorten Phriction slug field to 256 to avoid key length issues in UTF-8 default databases

Summary: This is a stopgap patch until we fix T345/T327 properly, but you can't
add a 512-character key to a UTF-8 column because it creates an index on more
than 767 bytes, which is the key length limit.

Test Plan: Ran schmea upgrade script.

Reviewers: indiefan, btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley, xela, jungejason

Maniphest Tasks: T327

Differential Revision: https://secure.phabricator.com/D1391