Restore Sentry grouping while preserving Papertrail extra context

[EvaBardou]

Closes #1351

I tested the implementation with the following simultaneous calls to the logger:

logger.warning("A warning log for testing", extra_context="something meaningful...", more_context="XXXX")
logger.warning("A warning log for testing", extra_context="something else...", more_context="YYYY")
logger.error("An error log for testing", extra_context="something meaningful...", more_info="XXXX")
logger.error("An error log for testing", extra_context="something else...", more_info="YYYY")

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Here is how the new setup renders messages in the console, Papertrail and Sentry:

Console

Papertrail - mocked through syslog (with extra contextual information)

Sentry (grouped Issues with contextualized Events)

As you can see on the screenshot, even with the extra context (stored in Additional data as before) the messages are still well grouped in Sentry, 1 issue associated with 2 events in the 2 cases.