Make epic_issues relative_position migration more robust
What does this MR do?
If someone installed EE, then downgraded to CE before this column was added, upgrading to the latest version of CE will fail:
- We have a backport migration for the entire EE schema but the table
epic_issues
exists, just not therelative_position
column. - 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.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66923.