Add global signal (if `--gscontrol gsr` is used) to component plots

[tsalo]

Summary

I think it might be useful to include the global signal in the HTML report, and I was thinking that it could be treated as a component for the sake of the report. It would also be nice to see how much variance the global signal explains.

[tsalo]

Alternatively, we could just add brain plots and time series plots to the HTML reports in new sections, like we've recently done with the T2*/S0 maps and the adaptive mask.

We wouldn't get as good a sense of the amount of variance explained in that case though.

[tsalo]

I can plot the global signal map, but I wonder if it would make more sense to plot a variance explained map instead?