Create update template that matches text already used for updated sky maps/source classification

Currently when an update notice is generated in gwcelery we create an update-type circular that is identical to the initial-type, which is not useful because we never send out or use this for sending out update circulars. We should generate an update circular template that matches the text we actually send out and is flexible for updated sky maps/source classification at least.

Text should match https://git.ligo.org/emfollow/followup-advocate-guide/issues/30 and previous circulars like:

sky maps- https://gcn.gsfc.nasa.gov/gcn3/24987.gcn3

source classification- https://gcn.gsfc.nasa.gov/gcn3/24462.gcn3, https://gcn.gsfc.nasa.gov/gcn3/24489.gcn3

We should note that there is variance between these and we should be open to modifications, but at the same time have a standard template would be helpful.

Note that https://git.ligo.org/emfollow/followup-advocate-guide/issues/33 is an alternative/complimentary solution to this