Add the current visible region to alert maps

[martinjohndyer]

Also maybe the positions of the Sun and Moon would be helpful.

[martinjohndyer]

We highlight the visible tiles over the valid period in the visibility alert as of #31. We don't show the visible region in the initial alert skymap, but that's intentional.

However perhaps the Sun and Moon might be useful:

• Add the instantaneous location of the Sun and Moon to the event skymap.
• Add the extreme locations of the Sun and Moon to the visibility map, over the visible period (e.g. 3 days).
  • If this is too complicated, just add the position at the mid-point of the visibility period.