Add placeholders for remaining plots

[sbailey]

@ana-lyons please add placeholder code/figures for the remaining plots, including

• a function that returns the figure (for now returning a dummy light gray plot of the appropriate size with a title like "PLACEHOLDER: (name of metric)")
• placeholder divs in the HTML template
• the appropriate script/div variables generated and passed into the jinja2.Template.render() call

The goal should be that updating those plots with the real version should be just updating the function, without having to also update the HTML template and Template.render() call (which could cause merge conflicts with other changes).

I'm going to submit some other layout requests, but please make this ticket your top development priority to head off potential merge conflicts / coordination headaches with other work.

Cross reference this to the notes I distributed a few weeks ago (I don't have them in front of me), but I think the missing plots are:

• summary page:
  • sky brightness
  • hour angle
  • total summed exposure time per tile (current exptime plot is per exposure, which is also useful but different)
• per night pages:
  • transparency vs. time
  • exptime (per exposure) histogram, to match exptime vs. time plot
  • I don't think we need total exptime per tile on the nightly pages
  • sky brightness vs. time and histogram
  • hour angle vs. time and histogram

[sbailey]

Added in PR #31 and #32. Closing.