Strand input + definitions

[jsstevenson]

We may not need to include strand very often anymore thanks to updates by @korikuzma and @jarbesfeld to Cool-Seq-Tool, but insofar as we still allow it as input, we should think about what model to use for its definition and ensure that it's consistent at all layers of interaction. Right now, the backend API is doing a custom thing that doesn't even involve enums. We've noted elsewhere that our projects' strand definitions are kind of chaotic: cancervariants/gene-normalization#350

Finally, it would be nice to make the UI on this cleaner -- maybe a radio button or toggle rather than text input.