Add flask button to download sky map from URL and upload to external event

This MR adds a button on the emfollow dashboard that will download a sky map from a given URL and then upload it to the given external event.

This is needed due to a common issue of automatically downloading from Fermi failing if the sky map is not available in time, or if we need to grab a sky map due to some unforeseen reason as was the case of GW170817 (maybe we get a link from a GCN circular announcing a co-detection).

This MR partially addresses #416 (closed)