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.
```python
def __call__(self, form, field):
    """Check given form field."""
    doi = field.data
    if doi is not None and not self.regexp.match(doi):
        # no point to further validate DOI which is invalid
        raise StopValidation(self.message)
```

--
alternative condition if you find it more readable: `if not (doi is None or self.regexp.match(doi)):`