Expression plots should be interactive

[kimrutherford]

This is a long-standing request/complaint. We should at least see the gene name/ID when clicking on a spot, or maybe on mouse-over. Other details might be useful too, like the expression level.

This comment has an example of how to make an interactive plot that might work for us:
holoviz/holoviews#3332 (comment)

Low priority, but I wanted to make a note of that comment.

Current static version: