Update external_triggers.py/raven.py to use ligo-raven 2.0

There is a need to update the external triggers section in order to use the next update of ligo-raven 1.19. This new version will reduce latency and add some new features.

The changes.md of this new version : https://git.ligo.org/lscsoft/raven/-/blob/master/CHANGES.md#20-unreleased

This would mean making the following changes:

Pass event dictionaries to ligo-raven functions to reduce calls to GraceDB
Switch to using the GraceDB SDK API
Create separate path for MDC events to be injected/processed

Edited Jan 26, 2022 by Brandon Piotrzkowski

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