Add cron worker for cleaning up unit test tables

What does this MR do?

Solves #297008 (closed)

This adds a new cron worker that will run once daily and will do the following:

1. Delete ci_unit_test_failures records that have failed_at older than 14 days.
2. Then, delete ci_unit_tests records that don't have any remaining associated ci_unit_test_failures.

Queries and related DB info

The unit test tables have been around for more than a month now. As of May 11th, these are their current sizes:

• ci_unit_test_failures: 490_501 rows
  • If we were to run the clean up job today, around 374_251 will be deleted.
• ci_unit_tests: 106_158 rows
  • After the ci_unit_test_failures have been cleaned up, around 76_299 will be deleted.

Here are the queries tested on #database-lab. Please note that these queries are executed in a loop.

This is the query from using Ci::UnitTestFailure.deletable:

SELECT "ci_unit_test_failures"."id" FROM "ci_unit_test_failures"
WHERE (failed_at < '2021-04-27 11:42:42.228774') LIMIT 100

Query plan: https://explain-depesz.postgres.ai/s/5g

This is the query from using Ci::UnitTest.deletable:

SELECT "ci_unit_tests"."id" FROM "ci_unit_tests"
WHERE (
  NOT EXISTS (
    SELECT 1 FROM "ci_unit_test_failures"
    WHERE (
      ci_unit_test_failures.unit_test_id = ci_unit_tests.id
    )
  )
) LIMIT 100

Query plan: https://explain-depesz.postgres.ai/s/bv

I also tested the actual delete queries, for example:

DELETE FROM "ci_unit_test_failures"
WHERE "ci_unit_test_failures"."id" IN (
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100
)

Query plan: https://explain-depesz.postgres.ai/s/tF

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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