Constants are duplicated between frontend and backend

[keysmashes]

Example:

frontend/src/constants.js

Line 41 in ae6e45c

export const maxFilesize = 31457280

Having the filesize configured separately for the frontend and backend will just lead to pain if 30MiB is ever deemed insufficient; there are several other constants (e.g. the grade descriptions) that should really be single-sourced as well.