`/api/v2/metadata`: include journalist accounts

[cfm]

• After Implement v2 journalist API #7604
• Towards [app] extend sync to journalist accounts securedrop-client#2560

The v2 Journalist API's sync endpoints should return Journalist user accounts. Unlike Source accounts and collections, these records are tiny and change rarely, but we should return them with the same versioning semantics so that we treat them as first-class objects and avoid special-casing them.

1. Make the v2 /sources endpoint also available at /metadata
2. Add a Journalist.to_api_v2() method
3. /index: Add a journalists key, keyed by Journalist.uuid
4. /metadata: Add a journalists key, keyed by Journalist.uuid