Enable import_details_page feature flag by default

What does this MR do and why?

Enable import_details_page feature flag by default.

Previous MRs for this feature:

MR	Status
!116090 (merged)	Merged ✅
!117160 (merged)	Merged ✅
!117671 (merged)	Merged ✅
!119912 (merged)	Merged ✅

Changelog: added

Screenshots or screen recordings

How to set up and validate locally

1. Go to http://127.0.0.1:3000/import/github/details?project_id=29 (use any project id that was imported).
2. Fake some failures in rails console:

p = Project.find(29)
f = p.import_failures.new
f.external_identifiers = {
  object_type: 'pull_request',
  title: 'A new pull request',
  iid: 18741
}
f.exception_message = 'Something went wrong!'
f.save

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #397650 (closed)