Make it easier to add new plots/tables

[dhmay]

Right now, adding a new plot or table isn't very easy or intuitive for a coder who isn't me.

I should add JamstatsPlot and JamstatsTable base classes, make all the existing plots and tables into subclasses of those base classes, and make it easy to add new plots/tables and control what gets displayed.

That last part should apply both to the web interface and to the pdf generation.

[dhmay]

@erevrav Thinking of you!

[dhmay]

@erevrav , I've done a mountain of refactoring to clean up the way plots and tables are defined, so that it's easier to add a new plot or table.

I need to communicate to you how to work with this interface, and we may want to make some more tweaks. But my number 1 priority is making sure that, in the process of all of this, I haven't caused regressions in any of the existing functionality. I need to carefully validate all three types of JamStats outputs: PDFs, the web interface, and the webserver interface.

I'd appreciate some help with this! Please check out the build here:

https://github.com/dhmay/jamstats/actions/runs/5435027352