Refactor including imports in stats queries

[RobertJoonas]

Changes

This PR prepares the codebase with a refactor in order to make SiteImport structs available in stats queries. The plan is to use SiteImports to decide whether or not scroll_depth gets included from imported data or not. This information will be included in meta.metric_warnings.

This PR also aims to move the logic of responding with imported flags (such as imports_included and skip_imported_reason) into a single place - QueryResult. The values for these fields depend on both query and comparison_query which this PR makes available in QueryResult.

Tests

• This PR does not require tests

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI

[zoldar]

LGTM, though I'm currently not super familiar with the query processing 😅

One thing I think would be worth considering is that preloading optimisation on plug levels at least.