Allow project import even when `shared_runners_enabled` conflicts with its group

Problem

About the new feature allowing to enable/disable shared runners and project import, If shared runners are disabled for a group on destination (or on a gitlab.com parent group), it does not allow projects to import, mainly because field shared_runners_enabled=<boolean> conflicts on import (by just being part of the exported archive) and returns:

"import_error": "Error importing repository into <group-full-path> - Validation failed: Shared runners enabled cannot be enabled because parent group does not allow it"

• being rescued on this line - http://gitlab.com/gitlab-org/gitlab/blob/b228b1568657c2a5d5724e6c7eca899bd47193e2/lib/gitlab/import_export/importer.rb#L32-32
• some issue importing file - http://gitlab.com/gitlab-org/gitlab/blob/b228b1568657c2a5d5724e6c7eca899bd47193e2/lib/gitlab/import_export/importer.rb#L48-48
• validation introduced in !36080 (merged)

Proposed solution

• Keep project value if Allow projects and subgroups to override the group setting is set, as it already does
• Otherwise inherit group setting of disabled, if a project has enabled set
• Keep project setting of enabled/disabled, if group setting is enabled.

Workaround

Either:

• Ensure that shared runners is enabled on the parent group on the destination GitLab instance. Then retry the import.
• Disable shared runners on the project on the source GitLab instance and then export the project again. Then retry the import.