feat(sns): Add the upgrade_journal to the get_upgrade_journal SNS-Gov endpoint

[anchpop]

The idea is for the upgrade journal to have all the information necessary to understand when and why the SNS got to its current version. This PR adds the journal to the API (but doesn't yet populate the journal, generally).

A follow-up PR will use the new endpoint in tests, and populate it in more places. As a proof of concept, I've made refreshing the upgrade steps add an entry to the journal, although the purpose of this PR is just to implement the API.