Rebuild iterations automation index

What does this MR do?

Rebuild a failed index on .com re #!62348 (comment 598513206)

Because this might have been a .com specific issue the migration checks if index already exists and is valid and skips migration if so, otherwise drops the invalid index and re-builds it one more time. 🤞

UP

== 20210611100359 RebuildIndexForCadenceIterationsAutomation: migrating =======
-- transaction_open?()
   -> 0.0000s
-- indexes(:iterations_cadences)
   -> 0.0034s
-- current_schema()
   -> 0.0002s
-- execute("        CREATE INDEX CONCURRENTLY cadence_create_iterations_automation ON iterations_cadences\n        USING BTREE(automatic, duration_in_weeks, (DATE ((COALESCE(\"iterations_cadences\".\"last_run_date\", DATE('01-01-1970')) + \"iterations_cadences\".\"duration_in_weeks\" * INTERVAL '1 week')))) \n        WHERE duration_in_weeks IS NOT NULL\n")
   -> 0.0372s
== 20210611100359 RebuildIndexForCadenceIterationsAutomation: migrated (0.0519s)

DOWN

== 20210611100359 RebuildIndexForCadenceIterationsAutomation: reverting =======
-- transaction_open?()
   -> 0.0000s
-- indexes(:iterations_cadences)
   -> 0.0038s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- remove_index(:iterations_cadences, {:algorithm=>:concurrently, :name=>"cadence_create_iterations_automation"})
   -> 0.0058s
-- execute("RESET ALL")
   -> 0.0008s
== 20210611100359 RebuildIndexForCadenceIterationsAutomation: reverted (0.0123s)

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