Increase resolution of external sky maps

Currently the resolution for external sky maps can be extremely small, leading to strange artifacts when computing the combined sky map:

https://gracedb-playground.ligo.org/events/E1022613/files/agile_skymap.png,0 https://gracedb-playground.ligo.org/events/E1022613/files/combined-ext.png,0

We can see this in the case of sky maps with error radii of 5 and 30 respectively:

from astropy import units as u
from astropy_healpix import HEALPix, pixel_resolution_to_nside

error_radius = 5 * u.deg
print(pixel_resolution_to_nside(error_radius, round='up'))

error_radius = 30 * u.deg
print(pixel_resolution_to_nside(error_radius, round='up'))

16
2

This should increased by quite a bit, or we give some minimum acceptable resolution (256?).

https://git.ligo.org/emfollow/gwcelery/-/blob/4e365ffe864042aa8c0f4098d45b4301aa80ba80/gwcelery/tasks/external_skymaps.py#L381-382