Upgrade mdn-polyfills to 5.20.0 and url-polyfill to 1.1.12

Environment: It should be run in as many supported browsers as possible. At least: Google Chrome Mozilla Firefox MS Edge Safari Ghostscript. If it's not installed, you can execute the following commands from the console:  $ apt-get update $ apt-get install ghostscript   Testing instructions: Login as admin. Access to "Site administration / Server / System paths", set " Path to ghostscript " to '/usr/bin/gs' or the corresponding to your environment and Save changes. Check  a green check is displayed near the "Path to ghostscript" field. Access to "Site administration / Plugins / Activity modules / Assignment / Feedback plugins / Annotate PDF" and check "Annotate PDF" feedback plugin is enabled. Create 2 students. Create a course. Create an assignment with Annotate PDF feedback ticked. Enrol students into the course. Login as the first student and submit a submission. Login as the second student and submit a submission. Login as a teacher (admin) and grade submission. Confirm , that the grading app opens correctly. Confirm , that the navigation between students correctly updates the URL. Confirm , that you can add annotations (comment, stamp, highlighting, lines) and successfully save changes.   Testing instructions copied from  MDL-65700 (given this is where these polyfills seem to have come from). Note : Browserstack is helpful for testing with different browsers that aren't easy to get hold of.