Split `CommitStatus#for_project_paths` into two queries

What does this MR do?

Implements #336434 (closed)

Splits CommitStatus#for_project_paths into two queries to prevent cross joining to projects. That is a step towards database decomposition.

Plucking is not recommended, but it turned out in some cases that a necessary step to do. Here is a related Slack thread https://gitlab.slack.com/archives/CNZ8E900G/p1629271665120800

DB queries

Before

SELECT "ci_builds".*
FROM "ci_builds"
WHERE "ci_builds"."project_id" IN (
  SELECT "projects"."id"
  FROM "projects" LEFT OUTER JOIN "routes" ON "routes"."source_type" = 'Project' AND "routes"."source_id" = "projects"."id"
  WHERE ((LOWER(routes.path) = LOWER('gitlab-org/gitlab')) OR
         (LOWER(routes.path) = LOWER('gitlab-org/gitlab-foss'))));

https://explain.depesz.com/s/atSz

After

SELECT "projects"."id"
FROM "projects"
LEFT OUTER JOIN "routes" ON "routes"."source_type" = 'Project' AND "routes"."source_id" = "projects"."id"
WHERE ((LOWER(routes.path) = LOWER('gitlab-org/gitlab')) OR
       (LOWER(routes.path) = LOWER('gitlab-org/gitlab-foss')));

SELECT "ci_builds".*
FROM "ci_builds"
WHERE "ci_builds"."project_id" IN (278964, 13083);

• https://gitlab.slack.com/archives/CLJMDRD8C/p1629282959342300 (can't add it to explain.depesz.com, the responses fails with 500 error)
• https://explain.depesz.com/s/9EpS

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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