apl_date: Add apl_read/writedatechunk functions

Dates aren't encrypted yet, but they may be in the future
apl_date: Data/time API accepts const struct ptr
apl_date: Fix apl_readdatechunk bug

Was reading the entire struct instead just the time_t value
apl_date: Don't write/read size of date data segment

We'll need this only when we start encrypting the dates
apl_gradebook: Store creation date as apl_date struct