sentry: add ability to send user messages along

this actually was an existing feature but limited to run as part of the bug reporting workflow. since that workflow is now fairly hidden we need a new way to get data from the user.

the sentrypage now contains a feedback field.

this required some reshuffling on the backend side of things since we have two scenarios to consider: comment finished before trace, and trace finished before comment. this is insofar a complication as currently sentry feedback payloads must be associated with an event, so the only legit ways of transmitting the data is event&feedback or event followed by feedback, but never feedback followed by event. headache material.

the primary change regarding this is that the event_id from python is now overridden by an envelope event_id. the latter now provides a stable id for both event and user_report instead of the previous nonesense of extracting the event_id from the event to serve as envelope id.

additionally we can now write the same envelope twice. this allows us to get away with the original UX - the event sends ASAP. a possible user_report will then be submitted later by simply re-delivering the entire envelope including user_report (the excess event is discarded server side).

Please not that for testing with kdesrc-build you need to make sure that your system level services are in order. Specifically the following files must be in place on the host:

/usr/lib/systemd/system/systemd-coredump@.service.wants/drkonqi-coredump-processor@.service
/usr/lib/systemd/system/drkonqi-coredump-processor@.service