Journalist API v2: generalize hint/shard/batch logic

[cfm]

@legoktm in #7770 (review):

[I]t feels weird/unnecessary that we're returning all the journalist metadata all the time, but I think it's probably negligible given gzip + how few journalists a system has.

I thought about this too. In fact, I originally included a journalists hint, but that felt weird/unnecessary too: unnecessary to shard such a small dataset by UUID prefix; weird to include a hint if it wouldn't affect either sharding of that dataset or the size of the overall dataset.

My instinct is that this isn't worth changing until something like freedomofpress/securedrop-client#1182, when we might as well generalize the hint/shard/batch logic across the /index and /data endpoints so that it's generic for all objects and doesn't special-case any of them.

Originally posted by @cfm in #7770 (comment)

This will be a new API_MINOR_VERSION with or without freedomofpress/securedrop-client#1182, because it will change the shape of a hinted /api/v1/token response. But it's mostly a server-side refactoring so that the /api/v2/{index,data} endpoints query objects the same way to build their responses.