Add keyboard shortcut to move to file permalink `y`

What does this MR do?

When viewing a file, pressing y will change the URL to a permalink that includes the full commit sha

http://localhost:3000/gitlab-org/gitlab-ce/blob/master/.eslintignore ->

http://localhost:3000/gitlab-org/gitlab-ce/blob/8ab94120ee0a87c7b1158ebafea101e3952ec758/.eslintignore

---

Previously, pressing y would copy the commit sha to the clipboard, 8ab94120ee0a87c7b1158ebafea101e3952ec758. This shortcut was not documented.

• MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4496
• Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/14470

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

• How do I get tree_join in scope for use in the rspec tests? (using it over File.join because that is what the haml uses)
  • See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8799#note_22090530

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together
• EE MR
  • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1176

What are the relevant issue numbers?

Closes #8082 (closed)