Allow for a variable number of tossups per packet

[Captain-Quack]

Decisions made related to improving the app:

• changed

   used_subcategories <- list(
       History = c(),
       Science = c(),
       Literature = c(),
       FineArtsOther = c(),
       RMPSS = c()
     )
  

  to

     used_subcategories <- lapply(categories, function(x) character())
  

  because it looks nice
• To avoid complicated data partitioning, the users input is automatically rounded down to the nearest multiple of 4

Decisions made related to not knowing enough R:

• deck refreshes when it runs out of samples
• generate_packet_TUandB just shuffles all bonuses instead of the related quarter (see inner comment)
• To avoid complicated data partitioning, the users input is automatically rounded down to the nearest multiple of 4