Improve cacheability of assignfeedback_editpdf/stamps

1) Create two different assignment instances 2) Create two test students users 3) As both students submit an assignment (pdf to be easiest) 4) As a marker grade both assignments, and use the stamp tool to add multiple stamps to both submissions 5) Open the dev tools and reload the marking UI, go to the network tab and then filter for the stamp urls, do this for all assignment / student combinations 6) Confirm that all the stamp urls for the same stamp have the same identical url (eg the cross stamp is always the same url) 7) Confirm that the url is served with immutable + cache headers and with a far future expiry