Add ability to add SANs to CSRs.

Unfortunately as the current version of PyOpenSSL does not provide the
ability to read the extensions of a CSR, we are unable to display the
SANs on the detail page.
I always advocate for list comprehension.

What's wrong with:
```python
sans = [
    domain.strip()
    for domain in self.cleaned_data['subjectAlternativeNames'].splitlines()
    if domain.strip()
]
```

I know that we repeat ourselves with domain.strip() but it's more readable for me. And it doesn't change the time complexity of the algorithm.

Another thing: `filter(bool, iterable` is the same as `filter(None, iterable)`.