TODO for Nov 30 before pre-testing

[Ashley1014]

@Ashley1014 @aunarang

• [x] test no_revise_go_back_no_clear for R1, R2
• [x] adapt no_revise_go_back_no_clear for cost/emissions
• [x] discount mpl_full prices for disc version
• [x] change dec num for disc (74 for all a and 0 for all b)
• [x] set r3_yes_keep variable for disc version
• [x] clear selection for disc version (set the back_revise variable for mpl_disc)
• [x] disable back button for three rounds