v2.1.19

Changelog:

• Book link on Teacher Features page is now a modal linking to all books. Chemistry, GCSE and Quantum Mechanics Primer books now have proper index pages at /books/chemistry_16, /books/phys_book_gcse and /books/quantum_mechanics_primer respectively.
• My Assignments page now supports #older and #completed URL parameters.
• Swapped position of differentiation and integration in the question filter to reflect order of study.
• Suggested questions on Fasttrack questions are now shown in recommended order, rather than a random one.
• Fallback to using CDNjs for MathJax whilst our own CDN is down.
• Fixed caching of groups, so that renaming a group takes effect immediately.
• Fix that requesting to show "My Booked Events" whilst not logged in caused an error on the server.
• Use the rounding scheme "half away from zero", our previous rounding which was implicit and happened to be "round half towards positive infinity" had affected some questions involving negative answers.