Feat/ledger

[Paulijuz]

After more than of year of waiting the ledger (money) system is finally here!* 💵 💸 💰

This includes:
✅ A flexible double entry ledger system for tracking transaction (deposits, payouts, purchases, etc.)
✅ Simple logic to calculate balances (goodbye ginormous sql query)
✅ Uniform ledger account interface between groups and users (no more source and drain accounts)
✅ Payments with Stripe (integrated with webhooks like before)
✅ Card detail saving with Stripe (makes use of new-ish CustomerSessions API for simpler integration)**

⚠️ *This PR is marked as draft until #443 is merged as it builts on top of it. I also have to tie up a few loose ends regarding permissions, although I think it might be best to do that in another PR.

⚠️ **I have not tested this fully yet so a few changes might still come.