Feature timecourse preview

[adelavega]

The idea is to add more ways to preview features prior to creating an analysis

1. In the Browse Predictors section, add a preview to each predictor that is essentially Figure 1 from the paper. A preview of the time course, with selected high/low frames previewed. It would be instantiated with a random dataset / subject / run selection, and could replace the current Dataset section

2. In the analysis builder, replace the "selected" tab with time course previews + correlation previews between the predictors

possibly in the future also add a "compare" page where you can arbitrarily add predictors outside the analysis builder.