Migrate "cancdn" to "canCDN" in the database

Summary: Ref T5884. We migrated with "canCDN" and then had live writes with "cancdn". Move everything to "canCDN" for consistency.

Test Plan: Ran migration, verified DB only has "canCDN" afterward.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5884

Differential Revision: https://secure.phabricator.com/D10273