Create index on environments by state

What does this MR do?

This adds an index on environments (project_id, state), motivated by the plans posted in https://gitlab.com/gitlab-org/gitlab-ce/issues/27644#note_190195075 (cc @krasio ).

On the database-lab instance, this brings down query runtime from about 300ms to 22ms:

SELECT
    COALESCE(environment_type, name) AS folder,
    COUNT(*) AS size,
    MAX(id) AS last_id
FROM
    "environments"
WHERE
    "environments"."project_id" = 13083
    AND ("environments"."state" IN ('available'))
GROUP BY
    COALESCE(environment_type, name)
ORDER BY
    folder ASC
LIMIT 20 OFFSET 0

Plan before: https://explain.depesz.com/s/BNZV Plan after: https://explain.depesz.com/s/X2Rb

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• [-] Separation of EE specific content