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.
@kneczaj IMHO I didn't change the behavior. Please check:
```python
doi = None
check = lambda x: x == 'doi'
doi is not None and not check(doi)
doi = 'doi'
doi is not None and not check(doi)
doi = 'doi2'
doi is not None and not check(doi)
```