Validate that template references are filled out

[matthiaskoenig]

I find references with:

    "date": "1000-10-10",
    "journal": "Add your title",
    "title": "Add your title ",
    "authors": [
        {
            "first_name": "Max",
            "last_name": "Musterman"
        }
    ]

Validation should check that

• date is not `1000-...
• journal and title is not Add your title
• Max Mustermann is not in authors

and give warnings to fill out the references