Refactor environment review out of test executions

[omar-selo]

Description

In order to support having and showing multiple runs or test executions per environment on artefacts we first need to take review concerns out of test executions. This PR does that.

Resolved issues

Resolves https://warthogs.atlassian.net/browse/RTW-365

Documentation

Web service API changes

• review_decision and review_comment have been removed from artefact builds endpoint
• all_test_executions_count and completed_test_executions_count have been replaced with all_environment_reviews_count and completed_environment_reviews_count in all endpoints that return an artefact for example https://test-observer-api.canonical.com/docs#/artefacts/get_artefacts_v1_artefacts_get

Tests

Added automated tests to backend and updated all tests as appropriate. Manually tested the frontend.