Integrate LitVar

[holtgrewe]

Is your feature request related to a problem? Please describe.
PubTator 3 is nice for genes. However, we don't have literature search for variants yet.

Describe the solution you'd like
Integrate LitVar via the API:

• create api/litvar module
• implement store/litvar
• create a SeqvarLitvarCard and integrate it with SeqvarView

Describe alternatives you've considered
N/A

Additional context

• https://www.ncbi.nlm.nih.gov/CBBresearch/Lu/Demo/LitVar/api.html

[gromdimon]

In progress