R1:5ada1211cdeb

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

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