Cannot use Bitbucket login and importer URL for the same OAuth2 credentials

To configure the new Bitbucket importer for OAuth2, you need to configure the Bitbucket Oauth2 settings with a valid callback URL (e.g. https://gitlab.com/import/bitbucket/callback). Otherwise you hit errors such as gitlab-com/infrastructure#908 (closed).

However, the Bitbucket OAuth2 login requires the callback URL to be blank.

Most OAuth2 providers allow you to specify multiple, valid callback URIs, but it seems Bitbucket does NOT support that:

From https://confluence.atlassian.com/bitbucket/oauth-faq-338365710.html:

If you do include the URL in a request it must be appended to the same URL configured in the consumer. So if your consumer callback URL is example.com/add-on the URL in your request must be something similar to example.com/add-on/function. If you don't include the URL in the request we redirect to the callback URL in the consumer.

Do we need to support adding a separate entry for login and import?

/cc:@vsizov @dbalexandre