Don't let parent accounts with children to be deleted

[argaen]

Currently the delete button is being shown for parent accounts that have children. The action of deleting is failing with FOREIGN KEY constraint failed but this shouldn't even be shown.

The condition for showing the delete button is in https://github.com/maffin-io/maffin-app/blob/master/src/components/pages/account/Header.tsx#L28.

EDIT: Apparently this is also for non parent accounts. I see accounts with splits have the delete button enabled? Need to fix this