Use new Vue-based ui for all import providers

What does this MR do?

This MR enables import UI for all remaining import providers (#26326 (closed)). This includes:

• gitlab.com
• bitbucket
• bitbucket server
• fogbugz

Local testing

Feature.enable(:new_import_ui)

Screenshots

Now all importers look consistent and use same components:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done