Implement items

[Morvvvv]

• Add items to project
• The following actions should be possible:
• [x] add item to bill
• [x] delete item from bill
• [ ] edit item in bill
• [ ] if bill is part of a group, set contributors to item
• split into smaller issues, maybe one issue per action

[GR0ZA]

More or less done, there is a new issue for remaining things.