Fix footnotes in user permissions table.

What does this MR do?

This is a documentation fix for http://docs.gitlab.com/ce/user/permissions.html.

The [string]: string syntax defines link labels in markdown, even if the link is never used. So the footnotes were parsed as link labels and cut out of the document. Now they are shown. Also replaced [^1]-style footnote marks with numbers in superscript.

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

No.

Why was this MR needed?

Two strings of source code are missing from rendered docs.

What are the relevant issue numbers?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together