Allow projects with dashboard as path

What does this MR do?

Allow projects with dashboard as path

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

Why was this MR needed?

There are a lot of projects with name dashboard. Now it gives people 404 because its reserved. I believe we can whitelist this name because we use dashboard only for top level routing and won't conflict with it.

Screenshots (if relevant)

no

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug2fa I don't think we need tests for constant
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/25893