Prepare issues table to backfill work items for epics

What does this MR do and why?

This MR does the pre-requisite work in preparation for the backfilling migration of epics into work items: !143844 (merged)

We are adding a tmp_epic_id column and ignoring it in the Issue model. We are also adding a unique index on it. We need that in order to be able to bulk insert records into issues table for every record in epics table and use the unique index to avoid creating duplicate records.

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