Add Sersic size-magnitude plots to core object plots

Thanks. Jenkins passed. I forgot I had to add a PR for the drp_pipe v2 migration json because I’m adding two new plot datasets but it even helpfully told me which lines to add and worked afterwards.

I made a couple of comments on github but otherwise looks good.

Plots and discussion in this Slack thread.

I fixed a few related/adjacent issues along the way.