Move default brand title to a method

What does this MR do?

This moves the default brand title in AppearancesHelper#brand_title to a separate method, allowing EE to redefine it without having to redefine the entire #brand_title method.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8735

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8604

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer