RK-A2

Today, the last style used S, the last question number asked Q and the list of questions L are stored in browser storage. However, if user changes subject and/or grade, they are all lost.

Going forward, store them per subject and grade combination.

Minimal test case:
- user chooses g1 and s1
- other than changing grade or subject, may do a bunch of things (go through some questions, change style preferences etc).
- for g1 and s1, the values are S1, Q1 and L1
- user chooses g2 and s2
- may do a bunch of things (change grade, subject, go through some questions, change style preferences, close all tabs and windows of the browser etc).
- user chooses g1 and s1. At this point, the applicable style, question and list should be S1, Q1 and L1.