Make Page::Main::Login#sign_in_using_credentials gracefully bail out when user is already logged-in

What does this MR do?

This improves Page::Main::Login#sign_in_using_credentials to gracefully bail out when user is already logged-in.

This is an improvement that is need for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19555/diffs and other QA MRs to come.

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

I don't think so.

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?