🎉 Use select_for_update tag to update note balances when we save a Transaction to avoid concurrency issues

"safe" template tag is not made for TeX templates, it replaces ' with ' but & is a special character

Raise permission denied on CreateView if you don't have the permission to create a sample instance, see #53

When an user is registering to the WEI, it doesn't pay the membership + the credit amount. The credit amount is deducted instead