deposit: doi syntax validator improvements

NOTE the name of the validator was changed to DOISyntax validator
because of convertion to a class. 

@lnielsen-cern I think you use it, so you need to adapt the code.
You can create a compatibility wrapper and add `DeprecationWarning`.

```python
def doi_syntax_validator(form, field):
    import warnings
    warnings.warn("Please use DOISyntaxValidator instead.", DeprecationWarning)
    return DOISyntaxValidator()(form, field)
```