add moon location and phase to nightly skyplot

[sbailey]

On the skyplot of each nightly page, add the location of the moon at midnight on that night, and preferably draw its phase (full, partial, new...). Seeing where the moon is can help explain why the tile selection algorithm was avoiding a part of the sky.

The moon locations and phase might be calculable from routines in astropy. If not and/or if that is slow, it might be more effective to pre-calculate the moon at midnight for every night for the next 10 years and cache that in a dictionary in a json file that is just read and used.

https://github.com/desihub/desisurvey/blob/master/py/desisurvey/ephem.py may have some useful code, but overall that is trying to calculate more than we need here so don't just copy it in full.

[rdoshi99]

I am unable to figure out why opening a pull request for the skypath moon branch does not reflect all commits pushed to that branch. When I go through the code subpage from surveyqa into the skypathMoon branch and look at the history, I can see the most recent commit from a couple days ago. However, when I try to update or open the PR for those changes, it only reflects commits from 2 weeks ago. That is what happened in #66 (a closed PR because of this issue).