Step 1/3: Add GraphQL query to fetch single release

What does this MR do?

Performs some background refactoring/prep work for #262667 (closed).

Specifically:

1. Add a new GraphQL query (one_release.query.graphql) to fetch a single release
2. Refactor the existing GraphQL query (all_releases.query.graphql) to use a shareable fragment
3. Fix our fixture generator to handle queries that include fragments
4. Rename convertGraphQLResponse to convertAllReleasesGraphQLResponse, and add convertOneReleaseGraphQLResponse

Screenshots

This MR makes no user-facing changes.

The Big Picture^TM

• Step 1 (this MR): Add a new GraphQL query to fetch a single release
• Step 2 (!44779 (merged)): Convert the page to use GraphQL
• Step 3 (!45170 (merged)): Enable the feature flag by default

Related to #262667 (closed)