feat(attributes): Add backfill button in flat and classbased allocation modes

Partial implementation of a "distribute points now" button for backfilling accrued Attribute points. Works with flat and classbased allocation so far; taskbased will require much fancier handling.

Requires [this commit in habitrpg-shared](https://github.com/HabitRPG/habitrpg-shared/commit/33222e0f1545afb1c51a68dc185631ce32a519ce).