Prepare for graph v2

[RobertJoonas]

Changes

Preparation for migrating the main_graph to API v2.

• Globally rename date interval to day to be consistent with the time:* dimensions in the new API. Docs update in Stats API is pending: https://github.com/plausible/docs/pull/530
• Stop returning the used interval from the main_graph controller action. The frontend already decides the interval based on what is valid, and the backend never overrides it - just validates it.
• Make it possible to pass a date param in API v2, relative to which the date range will be constructed.

Tests

• [x] Automated tests have been added

Changelog

• [x] This PR does not make a user-facing change

Documentation

• [ ] Docs have been updated

Dark mode

• [x] This PR does not change the UI

[github-actions[bot]]

|Preview environment👷🏼‍♀️🏗️ | |:-:| | [PR-4451](https://pr-4451.review.plausible.io)

[macobo]

In DMs I proposed a new DateTimeRange struct. I did some initial refactoring work for it here https://github.com/plausible/analytics/compare/prepare-for-graph-v2...prepare-for-graph-v2v2?expand=1, give it a look.

It isn't the full solution due to also needing some other flag on Query object to let code know that it should use relative time labels.