Add API route for sampled list of versions

[Mr0grog]

This adds a new API route at /api/v0/pages/:page_id/versions/sampled that gets a "sampled" list of versions — that is, one version per sampling period. Right now, the sampling period is hard-coded to one day, but we might change that in the future. The main idea here is to give people a list of versions to look at, but reduce the amount of data in a useful way for pages that we capture extremely often (e.g. https://epa.gov/ gets many captures every day). For most use cases, we don't want to look more granularly than each day.

Basically, instead of getting a list of versions, this gets a list of objects like:

{
  time: "2022-01-01",
  version_count: 3,
  // A version object like we would get from /versions.
  // This is the latest `different` version in the sample period
  version: { ... }
}

The order of sample periods is always latest first.

This is part of edgi-govdata-archiving/web-monitoring#161

[Mr0grog]

May not want to wait for this, since it will be messy, and for the endpoint to be useful we also need to call it from the -ui project. Probably not worth blocking that work on this.