Expand shared mutations page

[AdvancedCodingMonkey]

• Allow users to pick from any available variants in Shared Mutations (customize visible columns)
• Allow users to show different genes outside of Spike for Shared Mutations (this information is currently already in clusters.py but could be copied out to different files like it is for Spike currently)

[emmahodcroft]

Having a way to download the raw underlying data for shared mutations in some kind of table would also be great, so people don't have to go find this in a scripts file somewhere.