Improve RelSnip with Snippet Ranking

[victordibia]

• Currently, RelSnip provides lucene highlights to the reader. Can we further re-rank these highlights with BM25 query + highlight scores?. Explore implementation here and evaluate runtime costs. https://github.com/renatoviolin/Question-Answering-Albert-Electra/blob/master/rank_models/bm25_model.py

• enable this as a backend config.yaml feature only. show all parameters in front end on demand.