Fix default_project_creation already exists

What does this MR do?

1. A migration was backported by adding a new migration with a new timestamp. It worked fine because it checked column_exists?.
2. We had to modify this implementation to fix a weird failure for old versions https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27141#note_158604363
3. But we neglected the upgrade path for those who already ran the first backport implementation (devs, and 11.10 => 11.11). They would now get this error on next update:

PG::DuplicateColumn: ERROR:  column "default_project_creation" of relation "application_settings" already exists

This adds the column_exists? checks back to fix this for everyone.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team