Add immunomodulator / gene explorer module

[jaeddy]

Ideally would include some summary visualizations of cross-subtype and cross-study patterns (e.g., heatmaps, etc.) as well as ability to "drill down" and view underlying data and distributions. This link could be handy:
https://github.com/bborgesr/rstudio-conf-2018

It'd also be nice to think about framing this as a "Gene Explorer" module to allow for the possibility of more general use. For example, if someone wants to include this module in their feature to explore a pre-defined list of genes (analogous to our list of immunomodulator genes), they'd be able to use our module rather than create a new one.

[andrewelamb]

This now has a boxplot for gene expression with the choice of immunomodulator genes, with groups being the same choices as the cell content module.

[jaeddy]

I'm considering this module "added" (subsequent changes to scope and organization can be addressed with new issues).