Add `first-released-at` key

[cassidyjames]

Problem

For the API, we need to track the first release somewhere—it makes sense to keep that data here with all the other app data.

Proposal

Add a first-released-at key to the JSON files that contains an ISO8601 datetime of the first time that RDNN was merged into master.

Prior Art

We previously used a database to store the first release dates.