"Edit in admin" link on proofreading assignment card is visible to non-staff

[joemull]

Problem

https://github.com/BirkbeckCTP/typesetting/blob/b641acb277e7f8e0ec392f9ae3473642a0396202/templates/typesetting/elements/proofing.html#L5

Solution

It should be wrapped in a staff check like this one:

https://github.com/BirkbeckCTP/typesetting/blob/b641acb277e7f8e0ec392f9ae3473642a0396202/templates/typesetting/elements/typesetter.html#L6