Characteristic mutations revision

[flaneuse]

1. Convert table into bar graph
2. Allow user to adjust the characteristic mutation threshold
3. Create widget to look up the prevalence of a mutation within a lineage(s)
4. Added sublineage heatmap for combined lineages
5. Fixed Delta + lineage queries / reports

2021-09-02: unfinished things that need adjustments from the backend queries of mutations within lineage(s) from either @gkarthik or @mindoftea:

1. Switch https://api.outbreak.info/genomics/mutations-by-lineage?mutations=S:E484K,S:N501Y to be a bulk query (returns a list containing {S:E484K: [...], S:N501Y: [...]} and https://api.outbreak.info/genomics/mutations-by-lineage?mutations=S:E484K OR S:N501Y to be an OR query (returns {S:E484K,S:N501Y: [...]}
2. Allow combination https://api.outbreak.info/genomics/mutations-by-lineage?mutations=S:E484K,S:N501Y&pangolin_lineage=AY.1%20OR%20AY.2
3. Figure out what to do about lineage+mutation reports: if we lookup "prevalence of S:P681R in B.1.1.7 + S:E484K", is that what we want? How do we encode that?