Add vue3_migration.yml to external provider importers

What does this MR do and why?

Adds the feature flag vue3_migrate_import_project_importers and config files to the external project reporters AND the project import history table.

Targets the entry points for the following importer tables (after authenticating with the provider or in the case of manifest uploading a file)

• Gitea
• Bitbucket
• Bitbucket server
• Fogbugz
• Manifest
• GitHub ('d previously merged !251474 (merged) for the GitHub importers. This MR modifies that feature flag to cover all the third party importers)
• and the Project importers history page (pages/import/history )

The bulk of the UI consists of 'status pages' such as import/bitbucket/status and import/manifest/status are light wrappers for the same component ImportProjectsTable reused by GitHub, and have been verified.

One small change made to the Import History page, unrelated to the migration likely but it seemed a quick fix. While the history table isn't mobile responsive (and I have not changed that), the Details button needed more space next to the Status column.

References

#629161

Screenshots or screen recordings

	Before	After
Sample importer (GitHub). Such as /import/github/status	Unchanged
Project import history at /import/history

How to set up and validate locally (OPTIONAL)

1. Ensure github (and other importers) are enabled in admin/application_settings/general#js-import-export-settings
2. Turn on feature flag vue3_migrate_import_project_importers
3. Navigate to /projects/new#import_project and choose a provider
4. Easiest importers to check are GitHub and Manifest (the latter doesn't need a sign in but requires a manifest file upload)
   1. For GitHub, this requires a github personal access token. Once authenticated, verify the table of your github namespaces loads properly
5. No token required - Navigate to the shared Import History page at /import/history

MR acceptance checklist

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