Use new import UI for all providers

Feature flag: new_import_ui

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24608 we introduce a vue import app.

Importing projects from different repo providers requires different strategies, because of this, there are multiple view implementations. This is a lot easier to maintain with vue components.

The first iteration covers GitHub and Gitea, which share an API interface.

It would be great to get Bitbucket Cloud+Server next!

• GitHub - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24608
• Gitea - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24608
• Gitlab.com
• Bitbucket Cloud
• Bitbucket Server
• Fogbugz
• [-] Google Code - N/A, will be removed soon

/cc @stanhu @jeremy