Convert SavedQuery / NamedQuery to text, not bytes

Summary: Ref T1191. Similar issue to D10613. This column usually has a hash exactly 12 bytes long, but sometimes stores an internal builtin query name like "open", "all", etc. It might be nice to promote those to 12-byte hashes of a consistent length eventually, but for now just make this a variable-length column.

Test Plan: Ran migration, no longer saw issues with reordering builtin saved searches.

Reviewers: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10614