Make the revision on the `/help` page clickable

What does this MR do?

For quicker access, add hyperlink to the gitlab.com commits page for the current REVISION of GitLab.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • 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?

none

EE MR in gitlab-org/gitlab-ee!2089