Add date of last FSRS optimization to optimizer

[Danika_Dakika]

That’s been suggested before as well.

Show date when FSRS parameters were last changed

I try avoid optimizing FSRS parameters too often, especially since some instabilities were reported when parameters are optimized only after few reviews. But I also don’t want to mentally keep track when I last optimized them. Would it be possible to add some text or tooltip for the parameter field displaying the date when the parameters were last optimized (or changed manually)? Of course the metadata should also then be stored somewhere in the first place.