Upgrade markdown library to 1.6.0 for Moodle 3.1 release

Look at the help texts in the activity chooser and verify they look well formatted as before. Go to a course Go to Course administration ► Users ► Enrolment methods If it has no enrolment methods add one Click the delete button next to an enrolment method Check that the message is displayed correctly "You are about to delete the enrolment method "X". Are you sure you want to continue?" with no formatting Go to Site administration ► Appearance ► HTML settings Check that the settings description (down the bottom) is formatted correctly (with bullet points) Go to Site administration ► Appearance ► Themes ► Theme settings Check that the description is displayed correctly, in a code box. Note that this description is HTML, but it is run through the markdown parser (markdown can have html directly in it) Run the Unit Tests.