Replace 3rd party library toastr by built in bootstrap v5 toaster system

[Trainbird]

Since Bootstrap v5 has its own system for message toasts, we might just use that instead.

• toasts are now fully bootstrap design

• minimal footprint (net 20 lines increase)

• same call structure as toastr

• basically the same behavior as before

• renaming toastr to toaster to not collide or confuse them

• removing toastr JS and CSS dependencies