Modularize Almanac Namespace transactions

Summary: Depends on D19318. Ref T13120. Ref T12414. Move transactions for Almanac Namespaces ("name" is the only meaningful one) to ModularTransactions.

Test Plan:

• Created a new namespace.
• Edited a namespace.
• Tried to choose no name, an invalid name, a duplicate name, and a name in a namespace I can't edit; got appropriate errors.
• Grepped for AlmanacNamespaceTransaction::TYPE_NAME.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13120, T12414

Differential Revision: https://secure.phabricator.com/D19320