If someone installed EE, then downgraded to CE before this column was
added, upgrading to the latest version of CE will fail:

1. We have a backport migration for the entire EE schema but the table
   `epic_issues` exists, just not the `relative_position` column.
2. The migration that changes the default (quite reasonably) didn't
   check if the column exists.

If the column doesn't exist, we can just create it with the correct
default.