Make a LedgerAccount mandatory for cookie corner articles

[supertom01]

Articles in the cookie corner are linked to an account in the ledger, for the finances.

These were at some point optional, but currently break the website if the board forgets to add an account to it. So it is better to make this mandatory.

When updating the model, please make sure that this is actually possible for all the existing items, and check whether everything can still be purchased.