add indication of n points plotted in scatter/skyplot

[rileythai]

the scatter plots and skyplot, to avoid memory issues both server and user side, do not render all the points. instead rendering only 10k (scatter) or 3k (skyplot) at most.

we need to thereby inform users, outside of plot settings menus, how many of the total points available are being plotted, or users may fall into plotting missteps.

a technical limitation is that this has to be done within the plotly FigureWidget for effect callback reasons

inside the plot setting menu too may be fine as well, but it is a little more tedious to get to/see.

also see #37