Refactor Bitbucket importer to use BitBucket API Version 2

What does this MR do?

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/19946

Screenshots (if relevant)

This MR needs the following permissions in the Bitbucket OAuth settings:

• []

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together