Rename Learned Cards to Learned Cards First Seen, add new Learned Cards graph

[matthayes]

The current "Learned Cards" graph should really be called "Learned Cards First Seen" because it only counts cards exiting learning phase for the first time. If a card is either relearned or rescheduled to be new and learned again then it won't be counted. This is useful but could lead to confusion in some scenarios because the term "Learned Cards" is a bit ambiguous without further explanation. Adding an additional graph that counts cards that were relearned or learned after rescheduling could could help make it more clear.