Refactor import_projects store

What does this MR do?

This MR refactors internal import_projects store. This allows us to unblock:

• pagination (related issues #222889 (closed) #233059 (closed))
• correct handling of "in progress" repositories #235708 (closed)
• preserving state of user input across different pages for future "import all" work #235703 (closed)
• switches to new format, which will be eventually used by backend #27370 (comment 379034091)
• tests, tests, tests everywhere!

This is just a very deep refactoring, a couple of new features:

• Uses new namespaces endpoint to load namespaces
• Stores all user input in Vuex

Screenshots

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