ClinVarCitation Count

While talking to medical scientists, they said they often used how often a citation was used on ClinVar to prioritse what to read.

We have ClinVarCitation already, and could annotate counts in queryset, eg if you are already filtering via clinvar_variation_id

```
from django.db.models import Count
cvc_qs = (
ClinVarCitation.objects
.filter(clinvar_variation_id=1560)
.annotate(citation_count=Count("citation__clinvarcitation"))
)
Out[24]:
```

Or display in template as:

```
clinvar_citation.citation_count
```