Run RAVEN every updated preferred event; prepare external_triggers.py for alerts

This new design of external_triggers.py achieves the following design goals:

• Upload a new coincidence far every time the preferred event is updated
• Only upload a combined sky map after a preliminary alert has been sent out (we should not consider this or space-time coincidence far for preliminary alerts since these require sign-off from GBM collaborators)
• Checks whether coincidence far passes threshold every time coincidence_far.json is uploaded (this check if forthcoming in another merge request)

Incorporation into preliminary and initial alerts will come in a later merge request.

Note that this requires the latest version of ligo-raven (1.15) since this version removed ligo-raven's ability to apply EM_COINC in order for gwcelery to do this.